A couple tricks you can do involve repairing disk permissions from disk utility, and then resetting PRAM from the boot screen.
Another thing you could try is making a time machine backup of your osx, and then erasing the osx installation and recovering it using your time machine backup.
I had a problem of random cpu hangs from time to time when I fullscreen'd a video (flash or file) or other graphical things like command zooming or during growl notification popups. I took it to the apple store, and the geniuses and I agreed it was most likely a motherboard problem, but 1 week and a new motherboard later, the problem still existed. I then erased my osx parition and restored it from my time machine backup, and my problem went away.
You mention your internet, but what exactly are you experiencing when you say your computer is being sluggish? Is your sluggishness only during web browsing?