'94 940 Turbo Rough Idle and Surging
 

Aside from the annoying steering shimmy I posted about here, my '94 940 Turbo has had a rough idle, mild hesitation, and surging since I bought the car in April '10. I already posted about this, but can't find my thread for some reason.

Anyway, during the hot summer months, the car idled too high (1,100-1,600rpm) consistently. On a couple of occasions during the summer, the car belched smoke out of the exhaust while idling very high for the first couple of minutes after startup. Since getting colder weather here in GA, the car now seems to idle normal to too low (600-800rpm) and vibrates enough to make the dash, door panels, and headliner buzz/rattle.

The engine has a weird surging problem during acceleration. The harder I accelerate, the more prominent the surging becomes. I noticed the rpms waver slightly while cruising (maybe by 50-100rpm) at all speeds as well.

The car only has 105k miles. I did a full tune up using higher end parts from Bugicord and Bosch and changed the air filter. My Volvo mechanic changed the fuel filter, replaced the coolant temp sensor, and throttle position sensor. None of this has eliminated the issues at all. I've already put about $2k work into this car. HELP!