ASD 2.5.8, which can be found if you google it, will do a lengthy RAM test and give a report on RAM, the logic board and other components. The thing will sound like a rocket (fans) for the 120 mins. or so it takes, but it is worth the trouble.
ASD will also recalibrate the processors if necessary.
Using iStat Pro, I found that new arctic silver made a big difference in core temperatures of a dual 1.8 G5 that came into my possession a couple of months ago. A G5 tower is a small hassle to disassemble, but not more than a hassle—i.e., removal and renovation of the processors is not as bad as it looks. You will need a decent toolkit and a disassembly guide, but those are out there if you go looking. You don't necessarily need one specific to the dual 1.8s (note that there were several versions of that machine), since the screw locations and specifications are the same.
