Try running Apple's Hardware Diagnostics (I think below is the procedure for 10.6.8):
"Restart your Mac, holding down the D key while the Mac restarts.
After your Mac restarts, you should see the Apple Hardware Test chooser screen. If you don’t, Apple Hardware Test may not be available on your Mac. You may be able to start Apple Hardware Test from the Internet. Reconnect your Mac to the network, then restart your Mac while holding down both the Option and D keys."
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.14.x,; iPhones, iPods and iPads galore!