I have the same problems.
Additionally the app slows down to a crawl over a 2 hour workout. In the end it would only display every other second on the workout and it takes 5-10 seconds for a button to respond.
I have paid a bit more attention to what's going on todayand it seems that it's only the graphics that slow down into uselessness. The beeps alerting you that the rest period is over come at the right time but the timer stands still at 1-3 seconds for 5 to 10 seconds depending on how long the app ws open. If I am so unlucky that the screen saver activates fater 5 minutes (I had to change the setting since jefit does not keep the screen active anymore) after a 2 hour workout I'm left with a 'dead' phone that will take several minutes to wake up. Once jefit is killed everything is working again. Since this seems to be related to graphics I guess that we are looking at an iphone app that was ported to android instead of developing a true android app. For now I'll put jefit away and find something that works.

That's on a Samsung Galaxy S3