Hi , I would like to help .
Does the G5 completely shut down ? If so it maybe over heating .
When the computer gets load on it from applications and gets hot does it shut down?? , also please check the running temperatures of the CPU cores are they similar in temp ? use istat pro to read the temps of each G5 CPU while under load and idle http://www.islayer.com/apps/istatpro/ there should be no more than 7 degrees Celsius c in the working temp , and they should not go higher than 85C when under load or the computer will shutdown . is this the issue ?
Also never use air in a can to clean the mother board . it will kill it . static fries circuits . please use a soft brush to clean , keep the Computer grounded . wear a ground strap on one rist when working inside the computer case . .
also using vacume can do the same as canned air , cause static .
If the problem is not over heating . we can move on to some other tests and repairs. ?? please let me know .