I've had this problem before with my own SN42G2. It was often a memory issue, I had thought. But it turned out to be an IDE connection issue. What I had to do was be certain that my secondary ribbon cable was securely seated. It was cause my system to not boot for a long time. As in, I spent a couple of days not knowing what was wrong. I took everything apart and out of my cube. I have onboard video and I tried getting it to just post with CPU and RAM. Nothing worked. Turned out that the ribbon connection was freaky. Also, if you are running to sticks of RAM, try switching them around. I actually found some success with this. I think it was more that the RAM was connected well, even though it was truly seated correctly. Anyway, good luck with it.
Minolta 24/2.8, 50/1.7, 135/2.8, 70-210/4, 100-300 APO, 500/8
Sony 18-135mm SAM
Tamron 28-75/2.8, 90/2.8 Macro, 70-300/4-5.6 USD, 200-400/5.6