Holding 'K' and advancing 1 frame at a time with 'L'

I love that I can hold down K and hit L or J to advance 1 frame forward or backwards, I hate that it doesn't work 80% of the time. Often it'll work for hours and then randomly just stop working, holding K and hitting L just plays the footage normally. Restarting FCP doesn't make it work, restarting the computer doesn't either, nothing seems to get it back to working correctly except blind luck. Does anyone else have this problem?

I'm using FCP 7 and Mac OS X 10.7.4