toyota problem

i have a 1998 tercel which is acting up when driving. It bucks and shakes for a bit then clears and is okay . if i down shift it goes normal until overdrive then the problem returns. I put tsp ejector cleaner in the gas tank it seems to help. This problem occurs at any speed and frequently.. can you help?? also i bought a obd2 code reader but i cant find the plug receptacle.. it is not under dash drivers side.can you help on this one too??? bruce

A bit late, but a search shows it to be on the far left of the driver's side knee bolster, behind the fuse panel cover. Hope this helps!

thanks to jdmccright for solving my toyota problem re. obd2 code reader bruce