Annoying....

[95GTS]

My truck kinda has a surging problem. It want's to surge, like backward and forward motion, while driving. If you go easy on the throttle and drive regular like then it does it. If you have a bit of a lead foot then it doesn't. The truck will surge forward and backward while driving down the road untill up at highways (not neck breaking but annoying). Not really hard but enough that you can feel it, almost like a waterbed, and the tach needle will fluctuate slightly up and down. Does it more around 1800-2100 rpms. I don't know if it's the transmission or the engine because the engine isn't "acting up". Also this transmission shifts pretty dang hard into the next gears. Could one of the previous owners have put in a shift kit/valve body I wonder. The guy that had it before me didn't say a thing about a shift kit. How does a stock L trans shift?
Now I'm also getting some kind of rattling or something from underneath the truck. Kinda sounds like about the middle of the truck on the passenger side. Could this be a cat heat shield or something? I'm going to get under there sometime and check it out but I wanted to get some ideas of what to look for.

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1994 Red Lightning