They could be right. Many shops are cooler than where the machine is at home. Try this:
1. Remove the case cover.
2. Point a fan at the innards.
My 533mhz P3 (windows 98) has been freezing up intermittently ever since I had the processor cooling fans replaced (old ones were making noise).
I have taken it back to the shop twice, and they cannot duplicate the problem. It works fine for them, and they say the processor is NOT overheating.
But when I get it back home, it locks up. I've tried plugging it into a different electrical outlet, to no avail. Any ideas?