jsでタイマー処理がうまくいかない

前提・実現したいこと

現在暗算ゲームを作成していて、レベル１を選択してスタートを押すとタイマーが機能するのですがレベル２又はレベル３を選択してボタンを押すとタイマーが機能しません。原因がわかる方よろしくお願いいたします。

該当のソースコード

javascript
1document.addEventListener("DOMContentLoaded",function(){
2var text1 = document.getElementById('text1');
3var text2 = document.getElementById('text2');
4var start = document.getElementById('start');
5var level1random1 = Math.floor(Math.random()*10)+1;
6var level1random2 = Math.floor(Math.random()*10)+1;
7var level2random1 = Math.floor(Math.random()*100)+1;
8var level2random2 = Math.floor(Math.random()*100)+1;
9var level3random1 = Math.floor(Math.random()*1000)+1;
10var level3random2 = Math.floor(Math.random()*1000)+1;
11var kaitou = document.getElementById('kaitou');
12var ans;
13var kai;
14var cnt=0;
15var level = document.getElementsByName( "level" ) ;
16var a;
17
18
19
20start.addEventListener('click',function(){
21     
22     for (  a="", i=level.length; i--; ) {
23          if (level[i].checked ) {
24                a = level[i].value ;
25               break ;
26          }
27     }
28     if(a === "level1"){
29          text1.value = level1random1;
30          text2.value = level1random2;
31          ans = level1random1+level1random2;
32     }else if(a === "level2"){
33          text1.value = level2random1;
34          text2.value = level2random2;
35          ans = level2random1+level2random2;
36     }else{
37          text1.value = level3random1;
38          text2.value = level3random2;
39          ans = level3random1+level3random2;
40     }
41});
42kaitou.addEventListener('input',function(){
43     kai = kaitou.value;
44     if(kai == ans && a === "level1"){
45         var lv1no1 = Math.floor(Math.random()*10)+1;
46         var lv1no2 = Math.floor(Math.random()*10)+1;
47          text1.value = lv1no1;
48          text2.value = lv1no2;
49          ans = lv1no1+lv1no2;
50          cnt++;
51       }else if(kai == ans && a === "level2"){
52          var lv2no1 = Math.floor(Math.random()*100)+1;
53          var lv2no2 = Math.floor(Math.random()*100)+1;
54          text1.value = lv2no1;
55          text2.value = lv2no2;
56          ans = lv2no1+lv2no2;
57          cnt++;
58       }
59     });
60    var count = 60;　//カウントダウンの数字を格納
61    var min = 0;　//分
62    var sec = 0;　//秒
63    var stp = null;
64    var i = 0;
65    
66  
67    function count_start(){
68        i++;
69        if(i === 1){
70            stp = setInterval(count_down,1000);
71        } else {
72            
73            i = 0;
74        }
75    }
76
77    //カウンドダウン表示
78    function count_down(){
79        if(count === 1){
80            var timer = document.getElementById("timer");
81            timer.innerHTML = "あなたの"+cnt+"問解けました。";
82            alert("TIME UP");
83            clearInterval(stp);
84        } else {
85            count--;
86            min = parseInt(count / 60);
87            sec = count % 60;
88            var count_down = document.getElementById("timer");
89            count_down.innerHTML = ("0"+min).slice(-2) +"：" + ("0"+sec).slice(-2);
90            
91        }
92    }　
93    
94    //ストップボタンクリック時のアクション
95    function count_stop(){
96        clearInterval(stp);
97        i = 0;
98    }
99
100    //リセットボタンクリック時のアクション
101    function count_reset(){
102        count = 60;
103        min = parseInt(count / 60);
104        sec = count % 60;
105        i = 0;
106        var count_down = document.getElementById("timer");
107        count_down.innerHTML = ("0"+min).slice(-2) +"：" + ("0"+sec).slice(-2);
108        text1.value = "";
109        text2.value = "";
110    }    
111    
112    //ボタンイベント制御
113            var start = document.getElementById('start');
114            var stop = document.getElementById("stop");
115            var reset = document.getElementById("reset");
116            start.addEventListener('click',count_start);
117            stop.addEventListener("click",count_stop);
118            reset.addEventListener("click",count_reset);
119          
120     
121
122
123
124
125});

html
1<!DOCTYPE html>
2<html lang="ja">
3<head>
4    <meta charset="UTF-8">
5    <title>暗算ゲーム</title>
6    <link rel="stylesheet" href="style.css">
7    <script src="anzan.js"></script>
8</head>
9<body>
10    <h1>暗算ゲーム</h1>
11            <p id="timer">01：00</p>
12            <input type="radio" name="level" value="level1">レベル１
13            <input type="radio" name="level" value="level2">レベル２
14            <input type="radio" name="level" value="level3">レベル３
15            <input type = "button" value = "スタート" id="start" >
16            <button id="stop">ストップ</button>
17            <button id="reset">リセット</button>
18
19    <table cellpadding="1">
20        <tr>
21          <td><input type="text" id = "text1" style="text-align:right"></td>
22          <td align="left" width="2"><font size="+2">＋</font></td>
23          <td align="right"><input type="text" id = "text2" style="text-align:right"></td>
24      </table>
25      <p>解答</p>
26            <input type="text" id = "kaitou" style="text-align:right">
27
28            
29
30
31   
32</body>
33</html>