My 1998 GS300 has been running rough for a few months. It's been lumpy and stuttering on acceleration like a misfire of some sort. I took it to a mechanic and they couldn't find anything wrong with it. They did a compression test and they said it was all within good specs, they did a smoke test for any air leaks and that came out good. we replaced spark plugs, wires, coils, changed injectors around, ocv and still nothing.

Scanned the car and these are the codes I got.
P1120
P1755
C1223
C1254

Any help and comments would be highly appreciated. not really smart when it comes to the mechanical side so I'm lost

 

You need to probably take it to a more capable mechanic. While 1998 wasn't yesterday, the car is going to need more than a compression and smoke test to properly diagnose problems. The codes exist to help troubleshoot problems. While changing spark plugs might be a fine guess, since there wasn't a misfire code listed (I don't think), I wouldn't have expected fresh plugs to fix your issue.

P1120 (google!) appears to be a fault related to the pedal position sensor - and given this is a DBW car (mostly) it could certainly create driveability issues. There's another code P1755 - looks like related to a transmission solenoid. Given the transmission is controlled by the ECU, which also manages engine power / output, there's a possibility this is impacting you.