FCP 5 won't stay open for more than a minute

Question

First I upgraded from Quicktime to Quicktime Pro to convert .avi's to .mov's. That worked for about three weeks then suddenly all converted files played without any sound. Now after editing an mp4 on FCP I decided to upgrade the media to avi, so I upgraded the mp4 to avi using VisualHub, whose output plays fine in Quicktime (with sound), but while editing it together in FCP I've started to have problems while trying to integrate FCP with LiveType. I have to save after every change to the sequence. After integrating a LiveType file into FCP I found FCP shuts down after running for a minute, regardless of what I'm doing with it.
I'm currently working on an iMac Intel Duo and hope this doesn't rise out of a non-universal version of FCP running on an intel processor (perhaps a problem with Rosetta). If so I might have to upgrade to FCP 5.1
Any ideas?

Answer 1

How would I check the pixel dimensions?
With the mp4 they were copied off of a dvd with a program called Handbrake (it copies DVD's onto your iPod), though often whatever I get out of Handbrake I also have to put through a program called moviepod (which converts media files into mp4 format and once the conversion's done it appears on your iTunes browser).
I recently purchased a DVR which can load DVD's into the computer through multiple formats but then the DVD was rented and I figured I just as well use the media I already had in my computer.
Oh, and by the way, Thank you for the help so far. Though I'm afraid there's no clear resolution in sight I appreciate your apt attention.
The avi media is just the mp4 media converted through VideoHub (I've had no luck with either .avi or mp4 files using Compressor). The problem with editing may rely in my choice of converting programs and not FCP at all (the truth is I don't know much about these programs, outside what I use them to do (convert media) ).