Hello all,
I just wanted to give an update to a post I made a long time ago...

symptom: idle was dropping too low (<600rpms) causing the engine to shake. When the car was put in neutral it idled slightly higher in an acceptable range.

Parts needed:
8mm socket, 8mm open ended wrench, 13mm deep socket, 13mm open ended wrench, straight screwdriver, hex wrench (unknown size). Carb cleaner. Chevron techron fuel injector cleaner or probably better, bg 44k injector cleaner.

Solution:
I can't be sure which of these fixed it:
1) Used chevron techron fuel injector cleaner. I used treatment for 12gallons in about 5 gallons of fuel to increase potency.
2) Adjusted throttle position sensor / 'idle' pin (instructions from volvo mechanic)
i. removed throttle body (3 bolts and throttle linkage bolt thingy, gasket is unlikely to tear)
ii. clean throttle body w/ carb cleaner.
iii. adjust 'idle' pin (its the little screw that contacts w/ the lever that controls the throttle valve) by first breaking it free by loosening the 8mm bolt and then adjusting it until it just contacts the throttle lever. THEN, turn it an additional 1/2 turn.
iv. adjust throttle position sensor. loosen the 2 hex bolts. Hold the throttle body up to your ear. Turn the position sensor until you hear a 'click'' THEN, tighten the hex bolts. THEN, work the lever back and forth so that you hear a click every time you open the throttle body. If you do not, repeat step iv.

Thanks for all your help and I hope this helps someone!