|Ask the seller if you can open the case & have a look inside...that's the only way to know if a dust clogged heatsink is the problem. Another possibility is a weak or overheating power supply.|
As for the CPU, the link shows a P4 3.0GHz/800MHz CPU with HT. The 3.6GHz *may* have been an optional upgrade but if you want to know for sure, boot the system & check what's listed on the POST screen. It's hidden behind the Gateway splash screen so you'll have to press ESC key to close that screen & reveal the hidden screen. Or you can just wait to boot into Windows, the right click on My Computer, then click Properties.