Wrote these tips quite a while ago and just realized I missed what is probably the most basic troubleshooting tip: Quit FCP and restart it. Doing a complete shut down and restart is also sometimes useful. And strangely enough there's something called the Guinness solution, shut down, UNPLUG the computer, go out for a beer (optional) wait 10 minutes or so and then plug in your computer and start up.
First and foremost is deleting your fcp preferences
Here's a link to an apple page on fcp troubleshooting tips. Included are instructions on manually resetting fcp preferences.
I prefer this automated method of deleting fcp preferences:
Here are a few relatively simple and fast troubleshooting steps:
Disconnect all peripherals besides keyboard and mouse. If you're using a bluetooth mouse and/or keyboard, disconnect and use wired ones if possible.
If you are working with material via network, try working with material locally. Disconnect ethernet and/or disable airport.
Zap your p-ram (restart holding down command-option-p-r for at least a couple of cycles)
Repair permissions (applications: utilities: diskutility).
Run Diskwarrior if you have it on all drives. Also, run diskutility: repair disk on all drives. To run either of these to repair your startup disk, you'll need to boot from a different drive. If you've got a clone of your startup drive, that's ideal. Otherwise, you may need to boot off of your OS install disk (for diskutility) or the DiskWarrior disk.
Make sure you have a minimum of 10% free space on all drives. It's probably a good idea to have more free space on your startup drive.
Create a new user account. system preferences: accounts. You may need to reassign permissions for your scratch disk and any other drives you want to save files to.
I strongly recommend creating a bootable clone of your startup drive when you have a stable OS/Application combo. This way, if things go south because of a software or OS update, or file corruption, you can quickly restore your system to it's working state. Please see the user tip on cloning your startup drive for more information.
Recently, I've seen a few problems posted (fcp crashing on launch) that apprarently were caused by a dodgy font. Haven't seen this myself but it can't hurt to do the following. Run Applications: Font Book. Select all fonts and run file: validate fonts. This will report any corrupt or problematic fonts. If any are reported, delete them.
FCP specific troubleshooting steps
Make sure all material is in a fcp friendly format. H264 is NOT a FCP-friendly format.
Make sure your project file is not much larger than 100 mbs. If it is, duplicate the project file and then delete old sequences to get your project file size down.
Make sure all material in your timeline matches the timebase of the sequence. If it doesn't use compressor to convert to correct timebase. The only possible exception to this is 24p material in a 29.97 timeline. FCP will add 3:2 pulldown, but this is not the ideal solution for best quality.
Make sure your sequence is fully rendered before export (may take a slight quality hit working this way over sending an unrendered sequence to compressor, but it is a good troubleshooting step if you're having problems with your export or output in compressor)
Make sure all imported graphic files are RGB not CMYK