FCE2 Canvas freezing

Question

I have searched Jerky playback and freeze frame and found nothing exactly like what I am experiencing. Please note the exact process I am trying to do worked flawlessly less than 3 months ago, (the last time I used the application.
FCE2 has suddenly lost the ability to render on the fly. It will give me approx. 2 seconds of action in the Canvas window then freeze frame with audio and play head progressing naturally. The moment the stop button or the space bar is depressed the Canvas image shows the play head frame and once again for only approx. 2 seconds there will be action then a freeze.
The process we are asking it to do is to composite 2 video clips A (with a stereo audio track attached) and B roll kind of stuff (where B doesn't even have an audio track) except with slow dissolves between them including some suspensions as the music may dictate. This was no problem the last time FCE was used in fact it was capable then of compositing 3 video tracks and a title track doing flawlessly exactly what it seems to no longer be capable of, at least without any of these glitches.
I have changed render and scratch disks; put the work on external and then internal HDs; reinstalled FCE2; and even archived and reinstalled OSX 10.3.9 and FCE2 seems to be overwhelmed with delivering a viewable picture to the Canvas in each case.
I can think of nothing that has transpired with my E-Mac in the 3 months since last using FCE2 successfully.
Please does anybody have any ideas?

Answer 1

Here is an addendum to the problem. I just discovered that double clicking on L and J does produce a render on the fly at twice speed as normally expected. How can it have the gumption or the time to process 2x in either direction and get clogged up at 1x forward or backward?