Wow, what an assortment of intel chipsets; 845G, 865G, Grantsdale, 945G, Q43, Q965. Are there any common add-on cards or peripherals? I assume that all images are 32-bit.
This is like you say, probably a previously-unrecognized bug, yet if not, here's some troubleshooting tips that you may or may not find useful. 
Dell's have been notorious for bad capacitors. Google 'Capacitor Plague'. 'Borderline' caps tend to settle down after warming up. Try pre-heating a dissimilar set of machines for an hour (or two) before the install process. If the BS show up, it's likely not simply a 'borderline' cap. Smaller form-factors are more prone, due to limited cooling capacities. Often the blown caps are under the main cooling fan, nearest the CPU.
Use a portable AM radio to tune in to a faint station, then turn on the power supply. This works great for testing Remote Controls too, if a digital camera isn't handy; just put the remote up to the radio & press a button. EMI harmonics (or dis-harmonics) at a very noticeable level can indicate some potentially serious problems inside the case. Ensure to use grounded cords, or this trick won't work so well.
Check for overheating components with a non-contact infrared thermometer. Temperatures above 55°C are to be avoided (@20°C ambient, dell spec for gx280 says 35°C max operating), as capacitor lifetime doubles with every 10°C decrease.
Oh yeah, many of the smaller caps on these boards may only be rated at 85°C, instead of the usual 105°C! This usually only comes in to play inside thermal-zones. Then again, when a power supply starts failing, it usually starts 'trying' to take other caps on the mainboard with it (excessive ripple-current is a killer too)
Power supplies are another potential vector. Wires leading from the power supply to the mainboard should be 18 gauge minimum for 350W power supply, & 16 gauge for a true 600W+ (usually not a problem with these OEM's). How clean is the power to the workstations?
How many hours have the machines been running from day one, total? Can you venture an educated guess?
If you are confidant that your HW is up-to-snuff, then ensure all BIOS's are all up-to-date.
