1992 Oldsmobile Ciera mechanic put used computer in and now

My Ciera was stalling alot and my mechanic said the car's computer was not communicating with the diagnostic machine, so he had to replace the computer before assessing my problem. He then got the codes to replace the Idle Air Control motor, which fixed the stalling problem. However, when I drove the car home, I noticed a distinct vibration in 3rd gear only. This car was shifting fine before I took it to the shop. I took it back to the mechanic and he said it's probably my tranny going out. I've done some research since then, and I believe it's got to be related to the ecm they put in. I asked for the old ecm back (to put it back in and see if 3rd gear is normal again), but they had thrown it away. Could it be that this ecm iisn't communicating with the solenoid, which isn't telling the torque converter clutch to lock/lockup. Hence the 3rd gear vibration? I'm thinking of just trying another ecm--or could this replacement ecm just need to be "tweaked"? Additionally, I noticed that my AC wasn't working anymore, a strange coincidence. But my main concern is the 3rd gear vibration. Thank you!