Make sure that you have the "Romeo" repos enabled, and install the updated MintMenu and MintUpdate. There were problems with the "shipping" versions of both in which they consumed a tremendous amount of resources. This should solve part of your problems.
Also, keep an eye on the "compiz.real" process. It's been known to randomly skyrocket and consume a vast amount of system resources, sometimes locking up a system completely. This is why I'm personally against "compiz-by-default" - user problems almost invariably trace back to compiz at the moment. Try disabling "desktop effects" for a little while, and see if that doesn't help, and report back