Dear gonzad:

It sounds like a misfire to me. It could be the stuff you put in your engine (which you probably didn't need, especially if you are using high test gas as you are supposed to). And maybe it melted some sludge that is clogging your new filter but this seems unlikely to me. I would run the tank down to 3 or 4 gallons and refill, and in the interim check all vacuum hoses and their connections (there is usually a diagram on the front cowel somewhere). The engine will misfire if it is getting too much air as well as too little gas. Also check the ground connection on the intake manifold, a little black wire coming out of the harness near the firewall. This unassuming little thing controls the FUEL INJECTORS! Check all other ground and electrical connections as well. The above items have long been known to bedevil 240 owners. After checking the above at least two times, then and only then reach for your wallet.

Good Luck
Len