I know it's a bit late now, but you might have been better off replacing one chip at a time and letting the machine run for a while to test.
You may have a bad chip, but more likely one didn't seat right. If one is inserted making marginal contact, heat expansion and/or vibration will make it lose contact after a while, and letting the machine cool will appear to solve the problem temporarily.
What worked for me when I had a similar problem...pull EVERY socketed chip and re-insert. (One at a time so you don't lose track!) Not a bad idea to blow out the socket with compressed air before reseating. This solved a spontaneous reboot problem with my A3000T, it'll now run for weeks at a time.
Even if this doesn't solve your problem, you might consider doing it as preventive maintenance as long as you have it open anyway. Not EVERY time you open the case, maybe more like once every couple years.
