Then I may suggest, that you still keep on eye on that machine, and see which one fails and which don't. If you've a lot of failed WU/results, then there might be a hardware error, most likely powersupply, RAM or Harddrive. An coliding error within these 3 hardwares will make your LLR application fail, and most likely crash totally, and should it complete the WU it will most likely have an outcome with a false residual. But try to run the torture test with mprime95, it helps reveal errors rather fast.
Also please remember, that LLR is very sensitive, more than any other programs to heat, power loss or frequenzy loss, why you may not ask, I just remember it from the time I had a problem with my old machine, which is now replaced, it had a RAM problem which made a lot of false residuals. Anyway good luck, on finding out why, it might also be BOINC related, at least this is what I have to offer.