miss fire and stumbling

Hello all, Ive been dealing with an on going problem. I own an 89 sub. just purchased from individual far away. Recently a stumble has been happening when I take off from a stop. And it has progressed to a back fire that I hear out of the intake. The truck idles fine, a slight drop in rpms when I put it in reverse but not bad enough to make it stumble. I replaced the fuel filter this morning and cleaned up the throttle body. It is not triggering the check engine light. Took it to O'reillys and scanned but nothing stored. When i watch the spray pattern of the injectors they both look uniform. But when I move the throotle feed by hand I can noticable feel and hear a roughness in the idle. Any thoughts?

when mys was doing that it was just time for a tune up no half a@$ a complete job try it

Your Throttle Position Sensor may be the problem. I just read something about giving a bad read just off idle due to wear in the linkage that actuates the TPS or some such thing. It made sense to me when I read it. Hope this helps.