The sensor for the fuel and ignition computers is way under the intake manifold. It is under the #3 cylinder intake runner, and pretty well hidden from view. It is very hard to see and work on with the manifold in place. If you can take an inspection mirror or something and get a look at it, it might not have been reconnected when the work was done on your car. That might explain the rough running.

You say the car overheated and the head gasket was replaced. Did the mechanics check the cylinder head for warpage too? In my experience the head will warp pretty quickly if the engine overheats. If yours didn't, you are very lucky.

And, by the way, would you care to guess what I am in the middle of doing to Brunhilde, my 1991 Turbo SE wagon? Yep, putting on a newly resurfaced and rebuilt cylinder head because of a broken heater control valve that let the car blow out all of its coolant. And, at the moment I can't find what I did with the two temperature sensors. Drat!

PS: the lesson from my experience is - DON'T use cheap MTC heater control valves, mine broke when it was only just over a year old. Pay the extra $$ for a genuine Volvo valve (about $60 at Tasca Volvo) and you shouldn't have such problems. The $60 is a lot less expensive than pulling a cylinder head and getting it rebuilt/resurfaced.
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1955 Human, Scott; 1991 745 Turbo, Brunhilde; 1990 745GL, Snuggle Bunny