Sorry none of the advice has paid off - it's one of those things which is difficult to sort out over forum without physically seeing it for yourself. One last ditch thought is, what temps is your cpu / system showing in the bios - It could be your bios might have setting that if cpu reach certain temp then it reboots. If it were running too hot and reaching this peak, it could be reason why you are getting reboots. In which case you would have to reseat heatsink, or could increase threshold that it reboots. Have a look at this. Running out of ideas here I'm afraid. Monitor your temps in the bios for a while (they'll appear lower as your cpu is not doing any work) and report back.
Any further thoughts from you troubleshooting veterans out there?