what you need is Tech Tool Pro. It’s a very good professional tool to detect hardware and system problems. To me it sounds that you have got bad ram, but this is only a guess. You have to run the tech tool via DVD on your mac.
The software is about 200 bucks, worth every penny!
You shouldn’t have a problem. Check the specs (Hardware) before updating and make a clean install (separate disc, new OSX, everything from scratch, copy the FCP files, leave the existing FCP6 files untouched)
How do you capture?
I would stick to the original format. The rendering processes when you have to capture all the material again takes longer. It depends what your show looks like. Do you have a lot of graphics included, do you have to do things in motion, etc.
If its a single show just cuts and dissolves keep the 1080p codec. The other thing is progressive to interlaced. What is your final output? If it is a dvdSD then you should go to dvcpro50 and finish in this fomat.