Looking for a little advice. 95 Jimmy w/4.3 engine. Been having off and on problems with performance. EGR was fouling badly. It's been replaced with OEM. Noticed a miss so I did the plugs and wires next (the Distributer and cap were replaced shortly before I got it). Still having problems. What happens is going down country roads after about 10-15 miles I lose all performance. Engine winds up and actually seems to run good except that I have no power. Haven't noticed this on highway miles and I can do 100+ miles without the problem on the interstate. No SES light BTW. And the issue never happens on startup. Only later. Today, coming back from up north I experienced the issue again. Thinking maybe it could be computer related I pulled on a sideroad, parked, and shut her down. As it turned off I heard what I thought was a popcan clank on the ground. Weird. Got out and looked. Nothing. No litter. Fired back up, ran great again, and made it almost home (another 6 miles) and it ran bad again. After I pulled in the driveway I made sure to kill the radio before I killed the truck. Sure enough! Another popcan sound. I'm thinking I have a baffle in the muffler moving around and constricting airflow at times. When I shut it down the backpressure that has been keeping the baffle stuck in the wrong spot goes away and thus my problem disappears until I start going around corners and such again. Thoughts? Am I nuts?