Hello Nerd689

Welcome to PCHF
Well let's see. you have done half of the work so far. You pretty much have but to test other peripheral devices attached to the unit, or a new power supply.
Pretty much what I would do at your point is unplug every peripheral attached to the unit (modem, network card, video card, etc.)
I'm guessing you already ran a hard drive diagnostic and memory diagnostic, so I guess it is unnecessary to worry about those.
If you dare to venture further, you should try testing a good power supply (if the tech didn't do it already). Best bet is to pick one up at a local retailer that has a return policy in case it isn't the power supply.
If taking out all the devices and a testing a new power supply doesn't solve the problem, then that pretty much narrows it down to a bad motherboard. Most likely a defective power connector or North/Southbridge.
Order this way:
1. Run hard drive/memory diagnostics on current HD/memory (if not already done)
2. Pull out all peripheral devices and test unit to reproduce problem
3. If problem persists, leave peripherals out and test with a new power supply
4. If problem persists, suspect bad motherboard and decide whether a new unit is your best bet.
Let me know what happens
