Post effect glitches that weren't there before
 

Having rendered a mildly complex scrolling block of text at the beginning of a project (I returned to the beginning to add this later, not the same day I began the project) it looks great but there's now a glitch in the footage, it's no longer smooth - there's a slight jump just after the overlaid text leaves the viewer window.

Sometimes glitches like this are only evident during playback but when compressed into a Quicktime file everything is as it should be, however I can't selectively compress the opening 15 seconds of the movie (not to my knowledge anyway) and to compress the whole lot will take an hour, maybe more.

The compression starts at one minute and climbs into the 40's and then I just cancel it.

I have tried quitting and re-opening the project but alas no joy.

Any help greatly appreciated.