I don't believe this, but you're describing almost the EXACT problems I was having. Errors on expanding the install, Aero went bung a few times, it was only stable for ten minutes at a time... etc etc.
I just RMA'd my set of G.Skill PC3-12800 Ripjaws. I also went out and bought a new set of the same ram (so now I have two sets) and it fixed all those problems.
I have that memory... had been working great for a long time, then started getting wierd after I upgraded the ATI driver to 10.1 and after.... crashes, striped, etc.
After a lot of playing around, I discovered that the MB was using Command Rate 1, when the memory is rated Command Rate 2. Changed to 2... it's working perfectly now.
Memtest saw problems at CR1, none at CR2.
That's the dirty little secret of timing specs.. they almost never list CR numbers.