What would cause a 1998 Plymouth Breeze to start bucking when it is being drivin?

Something to do with the transmission, but that's all I know, sorry. My 1999 is doing it about 1 out of every 5 times I switch from 2nd to 3rd gear. It seems to have something to do with the rpm's at the time of the shift, since it always does it at the exact same parts of my commute (same stop sign, same straightaway, etc.) A new tranny is 1200 bucks so I am not thrilled about this... 102,000 miles, btw Had a bad plug wire cause that. Ran fine at idle and at a steady lower speed but would buck at highway speeds intermitantly and severely going uphill. Also, cam sensor caused that a different time but that should throw an engine code.