A few months ago I boxed up my Nforce3 system because it was not working properly and I didn't feel like tearing it apart again for a backup system. Now I have the time and desire to do so and this is what's happening. At various intervals, half of the time before completing startup checks, the computer reboots. Probably the max time between these restarts is about 5 minutes. Heat is not an issue, I seriously doubt that the memory or vid card is the issue. I've tried another ps, not the issue. Which leaves me once again to the mobo/proc question. My gut tells me its the damn motherboard, since I've had the proc in other machines with no problems. However, it been a while since that processor has been in another machine, but I don't see it going bad after less than a year of use. The MSI motherboards however have been a serious pain in my ass (both Nforce3 and Nforce4). Went through two RMA's on the 4 and its looking like I'll be going through one on the 3 as well.

If anyone has some insight on this problem, let me know, b/c I really do not want to unseat the processor from my 4 board to test the 3 proc since I've lapped the damn thing and do not want to do that again.

BTW, I suggest you don't buy MSI boards. After being a strong proponent of their Nforce 2 boards, it will now be a long time before I buy another one.

I had a similar issue, where mine seemed to be caused by the DDR2 memory. I beleave it was related to the timing of the memory interfering with the motherboards software because both the motherboard and the memory work great with different components. It was extremely low latency memory which seemed to be why it may not have worked.

I had this problem for 2 months and was cussing a storm up with DFI, BFG and tried 3 power supplies. So i know exactly how you feel. Boy was I pissed/relieved that it was something as simple as the memory latency.

Also try pulling your CMOS battery,
check it with a different video card (could be a short)
reflash the MB firmware,
check for any defects on the board.

I'm sure you've tried all of these but I ran Memtest86 half a dozen times before i'd even admit that it could be my memory and I could have saved alot of headache if I actually pulled components instead of just making assumptions.

I haven't tried pulling the cmos battery yet, but I don't think its the rest of the problems you listed. The Memory was tested in another mobo and had no problems. The video card I guess could be a problem but seeing as I don't have any more AGP boards, can't really test that. Just so its clear, the board resets even before boot is completed many times. If I can get it into the bios settings, it only stays on for maybe a few seconds to a minute once in there. According to the "D-Bracket" that MSI supplies, all initial checks are good as far as processor, etc. Just constantly reboots. I know MSI had issues with their cmos batteries not holding values, but I don't think this is the problem either. I'm leaing towards a bad component somewhere, though I don't know where.

I would check the system event log. If it is an XP machine, right click on My Computer then select manage. Navigate to Eventlogs, and system. These can be helpful