15 to 20 minutes after startup my Dell Inspiron 8600 slows to a crawl. The problem seems to roughly coincide with startup of the cooling fan. But after an hour or so the speed increases somewhat. Once started the fan operates continuously. The problem first arose within the last 5 hours of use and this laptop hasn't been used more than 200 hours use since new. But of course it is now out of warranty. Removal of a very slight amount of dust from the cooling fins next to the fan made no improvement. Any thoughts?