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Then it's a good bet the storage subsystem is not the issue and we can eliminate it.
Yes, memory was the pitfall with that architecture until Intel released San Clemente but that was late in the game.
If you've already ran a battery of tests, I can't really suggest anything else for you to try except removing the DIMMs and testing them individually, unfortunately.
Well, if I wasn't getting out of folding next month I would take you up on that offer since I have 6 systems that use FB-DIMMs. But, in such a hypothetical scenario, I would only be interested in 800MHz RAM because it's very hard to find around here, faster, easier to OC and apparently runs cooler.
It could be if there's a lot of heat blowing onto the memory modules. You said there's active cooling on the RAM, I doubt that would be a problem unless something occurred to the fans that rendered them less efficient. More likely it's CPU(s) overheating if the heat sinks are standard low-profile server models with standard cooling. Many servers aren't designed for folding in mind and use less than optimal HSF components compared to the enthusiast aftermarket.
From what I'm gathering, the RAM isn't really actively cooled, not directly in any case. I custom built ducts that direct room-temp air from the back of the case right to the modules. There is a fan at the duct intake (case back) and another fan at the end where the memory modules are. It might seem like overkill but in mid-summer the modules still get toasty and I run all my cases OPEN... This is what I meant by active cooling. Anything less than this and it's error city necessitating my overclocks a substantial decrease. Without such and other like measures, I doubt I would have maintained my top 20 position since '07.
Thanks for mentioning these details, it could make all the difference. I never run my systems, servers or otherwise, at anything less than 100% speed when folding. That includes the GPUs, all one dozen of them. Consequently, it sounds like a server room here but I'm guaranteed good OCs with dependable stability year round.
Please do because I'm much more certain that is the problem now. These systems really run on the hot side, especially with factory server cooling components and closed cases. They are simply not designed for 24/7 folding, unfortunately. If you discover heat to be the issue, lower the thread count one core at a time until you establish stability. Each time you lower the flag number, you should drop a couple of degrees.