python+kivyでボタンからの再帰処理（定周期）停止

Python + kivy でGUIデスクトップアプリを作成しています。

下記のコードで実行したところ、「Run」ボタンを押した後、画面が触れなくなり固まってしまいます。
「Run」ボタンで再帰処理を実行、「Stop」ボタンで停止がやりたいことです。
threadingを使ってマルチスレッドで処理しようとしている所のがまずいのかなと思うのですが、解決策が浮かびません。

python
1import japanize_kivy
2import webbrowser
3import time
4import threading
5
6from kivy.app import App
7from kivy.uix.widget import Widget
8from kivy.uix.floatlayout import FloatLayout
9
10from kivy.properties import StringProperty
11from kivy.properties import NumericProperty
12
13from kivy.core.text import LabelBase, DEFAULT_FONT
14from kivy.resources import resource_add_path
15from kivy.clock import Clock
16
17
18class MainScreen(Widget):
19    site_url = StringProperty()
20    count = NumericProperty(0)
21    text  = StringProperty()
22    interval_time = NumericProperty(0)
23    run_flg = True
24
25    def SiteAccess(self):
26        #webbrowser.open(self.site_url, 1)
27        print(self.count)
28        self.count += 1
29        time.sleep(self.interval_time)
30        Clock.schedule_once(self.update)
31        if MainScreen.run_flg:
32            th = threading.Thread(target=self.SiteAccess())
33            th.start()
34        else:
35            print("end")
36
37    def buttonClicked(self):
38        self.text = 'OK'
39        self.count += 1
40        self.site_url =  self.ids["text_url"].text
41        self.interval_time =  self.ids["text_time"].text
42        th = threading.Thread(target=self.SiteAccess())
43        th.start()
44    
45    def buttonClicked2(self):
46        global run_flg
47        MainScreen.run_flg = False
48
49    def update(self):
50        self.ids.text_count.text = self.count
51    
52    
53class TicketApp(App):
54    def __init__(self, **kwargs):
55        super(TicketApp, self).__init__(**kwargs)
56
57    def build(self):
58        return MainScreen()
59
60if __name__ == '__main__':
61    TicketApp().run()

kv
1
2<MainScreen>:
3    BoxLayout:
4        orientation: 'vertical'
5        size: root.size
6        BoxLayout:
7            size: root.size
8            pos: root.pos
9            size_hint_y: 10
10            Label:
11                id: label1
12                size_hint_x: 20
13                font_size: 16
14                text: "SITE URL :"
15                canvas.before:
16                    Color:
17                        rgba: .68, .73, 1, .5
18                    Rectangle:
19                        pos: self.pos
20                        size: self.size
21            TextInput:
22                id: text_url
23                size_hint_x: 80
24                font_size: 16
25                text: "http://reserve.tokyodisneyresort.jp/"
26                foreground_color:1,0.5,0,1
27                #canvas.before:
28                #    Color:
29                #        rgba: .68, .73, 1, .5
30                #    Rectangle:
31                #        pos: self.pos
32                #        size: self.size
33        BoxLayout:
34            size: root.size
35            pos: root.pos
36            size_hint_y: 10
37            Label:
38                id: label2
39                size_hint_x: 20
40                font_size: 16
41                text: "interval time :"
42                canvas.before:
43                    Color:
44                        rgba: .68, .73, 1, .5
45                    Rectangle:
46                        pos: self.pos
47                        size: self.size
48            TextInput:
49                id: text_time
50                size_hint_x: 20
51                font_size: 16
52                foreground_color:1,0.5,0,1
53                text:"5"
54                #canvas.before:
55                #    Color:
56                #        rgba: .68, .73, 1, .5
57                #    Rectangle:
58                #        pos: self.pos
59                #        size: self.size
60            Label:
61                size_hint_x: 60
62                font_size: 16
63                text: "(sec)"
64                text_size: self.size
65                halign: 'left'
66                valign: 'bottom'
67                padding: 10,10
68                canvas.before:
69                    Color:
70                        rgba: .68, .73, 1, .5
71                    Rectangle:
72                        pos: self.pos
73                        size: self.size
74        BoxLayout:
75            size: root.size
76            pos: root.pos
77            size_hint_y: 10
78            Label:
79                id: label3
80                size_hint_x: 20
81                font_size: 16
82                text: "access count :"
83                canvas.before:
84                    Color:
85                        rgba: .68, .73, 1, .5
86                    Rectangle:
87                        pos: self.pos
88                        size: self.size
89            Label:
90                id: text_count
91                size_hint_x: 80
92                font_size: 16
93                text: str(root.count)
94                canvas.before:
95                    Color:
96                        rgba: .68, .73, 1, .5
97                    Rectangle:
98                        pos: self.pos
99                        size: self.size
100        BoxLayout:
101            size: root.size
102            pos: root.pos
103            size_hint_y: 70
104            Button:
105                id: button1
106                text: "Run"
107                font_size: 68
108                on_press: root.buttonClicked()
109            Button:
110                id: button2
111                text: "Stop"
112                font_size: 68
113                on_press: root.buttonClicked2()