Dell PC spontaneously restarting, Mobo issue?

Hey all! I bought a Dell Vostro 430 CI7 about 3 years ago and last fall it started spontaneously restarting at seemingly random intervals. Having been in contact with Dell and found out my service tag has expired, I am on my own trying to fix this horribly frustrating problem.

The restarts happen on average 0-5 times per day (0 is extremely unusual) and it's NOT a blue screen or typical Windows error but a reset more as if the computer had suddenly lost power, kinda like a fuse just blew or something. BUT, it's also not completely instantaneous but always preceeded by a 1-2 full second freeze before it dies. Whenever I'm working on it and my mouse cursor suddenly freezes on the screen, I know the next second it will just go *blip* and die to reboot with a "Windows was not shut down properly" error. The anticipation for these moments have now become a bit like my own personal water torture session, saving like a madman (and still often just not frequently enough) and waiting for that next crash  

There are two reasons I'm *suspecting* it might be a motherboard issue, but of course I really don't know what causes this.

First of all, because I've soon replaced almost everything in the can except for the Mobo and Processor. I replaced the default PSU with a Corsair HX650W, the main chassi fan is gone and replaced now with 4 (!) extra fans to keep temps as low as possible. Main hard drive was last week proven faulty with damaged sectors and I finally thought I was on to the problem, only to have another crash maybe 5 hours after installing a new main drive. Also the default graphics card has been replaced with a Gigabyte gtx460.

Second reason, is the motherboard is the only thing showing suspiciously high temps. Right now during normal Windows 7 usage it putters away at 63 deg Celsius but if playing any demanding games or such, it quickly rises to the 80-90 interval (mind you this is with 4 fans and the chassi side permanently removed since awhile back. Yes I dust often   ).

Anyway, I know at certain points in the past BEFORE this problem occured, with the chassi closed and only the original single fan running, I've at some points seen motherboard temperatures of up to 102 deg C on this sucker. Even though it didn't cause any crashes at that time, I have a slight fear it could have done some permanent damage to the motherboard maybe making it more unstable or such? Strange thing is though, the crashes don't seem to correlate in any way with the temperatures. It can run fine for a long time at 85 C and then next day after 1 hour running at 60 C it suddenly dies several times in a row.

It also seems to die mostly when I'm on the computer working on it, actively doing stuff. When I leave it running over night it's pretty much always fine when I come back into the office in the morning, but can then often crash again after just half an hour or so of actively working on it.

Anyway, I've heard these motherboards (generally speaking) should be able to withstand up to 110+ C but I actually don't know specifically what mobo is mounted in this model per default so I can't look it up for sure. Pretty much all the specs *except* that were available where I ordered the computer. Does anybody know what make and model of motherboards Dell used for Vostro 430 CI7 (6 GB) three years ago and what it's max temp tolerance is? Processor is Intel Core i7 860 2,8 GHz 8 MB with P55 chipset.

I should also mention I've tested the RAM with memtest86+ after a reboot, but only for 2 passes (roughly 1.5 hours). Maybe I should run it longer just in case? I've heard certain memory errors can be more random and only pop up after a long time of continous scanning.

Sorry for a very long post but I wanted to detail the problem as best I could. I really hope someone could shed any kind of light whatsoever on this extremely irritating problem. Starting to run out of things to try, not to mention hope in general