Most troubleshooting points to a general list of how problems break down. While I have not done a spreadsheet, charting the numbers/percentages of these incidents, it seems that they breakdown in this order with the first two being the cause most of the time:
Because of the unlimited variables, that can be involved, it's impossible to say what will work for any one user, without full info. The System, for instance can vary from people using an underpowered laptop to people with full NLE workstations. The various equipment, OS's and settings have to be addressed on an individual basis. This ARTICLE will give you some tips on listing your System's specs., plus other tips for the rest of that list of possibilities.
In general, a good place to start is to update both the audio and video drivers. Even on a brand new machine, it's likely that these are several versions old.
1) Ensure you've got enough hardware resources. I wouldn't try to run this program on anything less than a dual-core processor with 4 gigs of RAM and at least 30 gigs of free, defragmented hard drive space. Don't try to edit AVCHD on anything less than a quad core machine.
2) Ensure you have the latest Windows Updates (manually go to Windows Update and get the non-critical updates also) and the latest version of Quicktime and your graphics cards drivers.
3) LET THE PROGRAM COMPLETELY START UP BEFORE YOU TRY TO EDIT! This program takes a long time to fully start up. Wait until it completely loads, all of the clips have thumbnails on them and your hard drive stops churning. (You can even open Task Manager and watch it until the processor calms down.) On a slower machine, it can take 3-5 minutes for the program to completely load and settle down.
4) Whenever possible, use only standard video formats as your source files: MiniDV, HDV (tape-based hi-def) or AVCHD. Watch out for video from things like pocket hi-def camcorders like the Flip.
5) Render often. (I also recommend you go to Edit/Preferencess/General and turn off Background Rendering.) Whenever you see red lines above your clips on your timeline, press Enter and let the program render them. (The red lines will turn green.)
As I've said, I'm now running version 8 on both an XP machine and a Windows 7 machine and it's quite stable with my current workflow.
Hope that helps!
This FAQ Article will give you general tips for optimizing your computer and your OS.
There are also several FAQ articles on specific features and issues unique to PrE 8. These are at the top of the FAQ seciton, accessed from the menu to the right of the main forum page.