We recently purchased a 2002 Buick Lesabre with 60,000 miles on it. During the purchase it acted great (of course!) The next day we noticed a loss of power during acceleration that comes and goes. When you start out it is fine, but almost randomly at points it stops picking up speed. Even if you floor it the car only maintains its speed. A few seconds later it will start to pick up again. After it does this the check engine light comes on. My first idea was that it had bad gas in it. The previous owners put it away each winter when they left for Arizona so it might have had old gas still in it. I put in some injector cleaner and filled the tank. Also I replaced the fuel filter. The problem seemed to go away, but the next day it was back! Any ideas on the cause? I am almost thinking the computer in the car is retarding the engine since it is detecting knocking, but there are no signs of that other than the lack of power. It is a 3800 engine. Thanks!