When software has gone wonky on me when it previously had worked just fine, I do a couple of things:
- update software/ os (which you have done -- incidentally, did problems start with Mac OS 10.7?)
- repair permissions using Disk Utility
- check to make sure i don't have RAM that has gone bad
- make sure my hard drive is working fine (use Disk Utility to verify/ repair disk)
- run virus scan
Those are my ideas so far, I'm sure other people have a more comprehensive list of things to try.
Usually, iMovie either works great or it crashes. In that case, I would recommend looking for incompatible QuickTime components like 3ivx. Or reinstall iMovie. Or reinstall OSX.
However, in your case, with a slow decline, it sounds like over time you have accumulated a large number of swap files, and these may be quite large. If this is the issue, you can fix it by shutting down and restarting your Mac.
I have 16 GB of memory, but I still run a free utility called Memory Stick. When I get over 6 or 7 swap files, or when the last swap file is 4GB or larger, I will reboot my Mac.
I know this sounds obvious, but it is the only thing I can think of that matched your symptoms. If you are in the habit of closing your laptop without shutting it down, this could happen over time.