I just bought my first Volvo, an 83 240GL. I got it mainly for my daughter to drive to college and work in. (wanted something long lasting and safe)This car is a joy on the highway! I'm hooked for life. Here's the probllem. On cold start the idle serges about 600-700 rpm. After a few minutes it settles down into a fairly consistant idle. But it shakes th whole car! Any slight throttle and it smoothes out. It run great at any speed. I've had it up to about 85 and smooth as silk. Also when you stop the car at a light or whatever, it doesn't stumble or anything but it does surge every few seconds. I have changed the plugs, put in new injectors (improved mileage) and the cold injector looks new. new air cleaner, all the basic stuff.
Idon't know all the terms for parts but have a manual. I can work on cars (77 Nova 367K mi, 84 Harley 176K mi) but haven't for a while Hope someone can give sometips and clues. Thanks OBTW this a great sight keep up the great work

Just some ideas:

Check for vacuum leaks.
Clean the idle air control valve.
Clean the throttle body.
Adjust the idle according to manual.
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Thanks everyone for the advice. 81 242 Brick Off Blocks, Turbo bars and wheels, B21F, M46; 86 244, B230, 148k , auto.

Thanks for the hints...I'm going to roll up my sleeves later today. Will let you know.