Game countdown timer skipping on replay! Help

Ok i’ve got a simple demo for a spot the difference game,
you click, find the differences, if you find all 5, the find count goes down to 0 and you get a win screen… that’s fine

if you don’t find them all within 30 seconds, it goes to a loose screen, that’s also fine

the problem comes when you click to replay and then seemingly for no good reason, the timer skips more than 1 number a second…

this is the code i’m using on the timer

count = 30;
countdown = function(){
if (count ==0){
doCountdown = setInterval(countdown, 1000);

you can see the game in action, and also the error when you replay (not refresh, that seems fine) here:

any help / suggestions would be great!