Yup, poorly seated video card or RAM are common issues.
Is that 8 GB a pair of 4 GB sticks, or 4x 2 GB? If it is 4 DIMMs, try removing two of them (run just one stick per channel). If it runs stable with half of the RAM, that's a big clue (marginal RAM or marginal timings that don't work well with 2 sticks per channel).
You're just going to need to work by process of elimination. If re-seating components and Memtest don't turn up anything, you may need to borrow spare components from someone, or see if you can test your components in a friend's machine.
If the world isn't making sense to you, you're either drinking too much or not drinking enough.