I vote it's your graphics card.
I recently built a brand new system, AMD Phenom II 965, Asus Mobo, ocz SSD, ATI Radeon HD 5670 blah blah. I went through the same stuff you are.. double/triple hardware combinations and drivers are up-to-date. I ran all kinds of memory tests, CPU tests, 3d benchmarks, you name it. But some games would cause an instant crash.. or it would reboot randomly while watching a movie. Heat wasn't the issue, that was my first guess for random failure. Spending 2-3 days on each hardware part, I ended up finding people with similar problems as me with the Radeon HD 5670. Turns out the new driver is fail and you have to use an older one. I ended finding a re-pack of sorts from ATI specifically for my card. Completely fixed.
PS: The only difference is i'm not running A/V.. i prefer a little nudity. But i do use sandboxie to run a lot of apps in. Including Firefox.
Opera had sidebar since...a long, long time; one if its defining features (might be different than what you're used to of course). Likewise synchronization, working for a bit more than just bookmarks; and across all Opera browsers (also Mobile; or Mini, the one for j2me phones) plus accessible via webpage. It has also integrated dev tools.
Color tabs - no (afaik...
The IBM 2250 is impressive ... if you compare it with a system selling for a tenth its price. -- D. Cohen