I believe adjusting the throttle plate stop screw will only make matters worse. That is what the PO did to my '93 940 and I had to go through all the steps in the manual to get it set right again. You might try the following:
1. Check for air leakage in the accordian hose going from the air mass meter to the throttle body. Take it off and examine it closely for splits in the edges of the pleats. I had this happen on a 240. I did a temporary repair with black plastic tape.
2. Remove and clean the Idle Air Valve. These can get gunked up and stick. The cold temperatures would make the sticking worse. The FAQ should tell more.
3. Clean the throttle body then adjust the throttle plate to factory specs. The idle in my 940 was hunting up and down and by carefully adjusting the throttle plate, I finally got it to be stable.

Read the FAQ for a good discussion on idling problems.

I don't know why the dealer put in a new AMM when the code meant an idle air control problem. It seems to me you and the shop should talk that over. A new AMM costs too much to be used blindly in a guessing game. When my AMM went bad, the car would barely start and had very little power since it was in the "limp home mode". The dealer may have cleared that 233 code and your car's computer then reset it during the time you drove the car home. Good luck and keep us posted on what happens.