hot- thats the key.
there is only so much heat that these things can deal with.
the ambient temperature is very important.
above a certain temperature - any computer will crash - or a system component such as a hard drive will fail.
The heat tolerance will be different in every single computer- so its not unusual for one to work and one to fail.
the rated temperature is 10C to 35C - but once you have a scsi drive and other cards in the case - this temperature will be lower. possibly less than 30C.
http://www.apple.com/powermac/specs.html
You will need to provide additional ventillation air-conditioning - or fans.