4th Gear Problem

Hi i just bought a lumina euro with a 3.1 engine with only 17k miles on it the previous owner never took care of it. when i took it home it was missing and when it got warmer it got worst. When i was driving it back home it would go into 4th gear the car would shake really bad. i got home and changed the wires, plugs and middle ignition coil. I started it and ran much better it would not miss or anything. now when going on the highway i feel the car shake a little like its missing or something when the car goes into in 4th gear around 50 mph. would really like to know what is causing this any info would be appreciated Ty

things i have change
put in AC Delco Wires and Rapid Fire Plugs
change the fuel filter
pcv valve
MAP sensor
Oxygen Sensor
Throttle Position Sensor
Ignition control module

Welcome to AF. Lots of new parts but the root cause still exists, this could be an engine misfire still, only under load in 4th or the tcc is shuddering, tough to tell without driving it, I would try some injector cleaner such as Seafoam or Chevron Techron, there is no easy diagnosis for this in an online forum.

Hey Ty for your reply i believe that the previous owner never bother to do anything about the 2 or more missing cylinders or even wash the car lol. I'll try injector cleaner tomorrow and let you know how it turned out. Ty Again