I'd still be a bit surprised if memory were the root of this. I feel it must be connected with the tape drive.
However, relevant or not, let me tell you of an experience of mine with our earlier server (Netware5). It would regularly run 200+ days without reboot. The only reasons were to replace a tape drive (!) or once because the power company wanted to replace the company meter !
However, suddenly I would come in in the morning to find the server had rebooted itself during the night, which you would not know if you didn't spot the elapsed server time suddenly becoming hours rather than tens or hundreds of days. This went on for two to three weeks, during which, like you, several things were tried without success. I finally decided to replace the power supply - quite expensive - but no change! Suddenly, it failed during the day, and the solution was found. It was the big, strong UPS, which always appeared fine, passed all the tests etc, etc. But, the batteries were, actually, shot. Why did it only fail at night ? No idea.
Replace that and everything was cured. Then we got a new server, where I fought a battle to stay with Novell, when all around me wanted SBS2003. For the first few months, I had to reboot at ten to twelve weeks running time because of falling free memory, and things were looking sad. But at SP3 Novell finally sorted the memory management, and it has since run for the hundreds of days at a time, as always expected. I have no reason to regret the decision to avoid Windows, except once software that was designed to run on SBS2003 had to be installed on a PC instead.