Oops, saw your update, glad you got it resolved!

When my car acted like this, it was the fuel pump check valve. Easy to get to and easy to replace.

http://www.fcpeuro.com/products/volvo-fuel-pump-check-valve-1587010539

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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 352,039 miles, will she make it to 400K ?? >>You haven't really worked on a car until you draw blood<< :-}

It's been a while but I thought I would post an update.

It was the fuel filter which was really old. The guy who replaced said he thought it was the original as
it had VOLVO written on it.

Got some new tires and replaced the filter and she runs great.

Although, I think the next fix may be the instrument cluster. Looks like it's a common thing for older
Volvos.

The fuel and gas gauge had stopped working - were both at the bottom. So I thought - uh oh - it's time for the cluster - when another guy at the shop hit the top of the dash above it and the gauges were working once more.

He was telling me that it was basically a common things and that the circuit board would probably be needed to be taken out at some point and sent out to be re-soldered.

It sounds like what I just did with my macbook pro when the video chip crashed.

Congratulations for getting most of the bugs out of this New to you car.
I think you already did all this.. basic, Clean the Throttle body, clean the idle control stuff. If you did all that , not sure where to point to. Is long accordian Bellows attached tightly to Throttle body and AMM?
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.lyse.net/brox/TonyPage4.html http://cleanflametrap.com/tony/

First thoughts, for me, would be a sticky Idle Air Control Motor or a dirty throttle plate, with its switch not adjusted correctly.
Not clicking the ECU from idle to running programs with the throttle plate. Either/or both, have to be in time with each other for smooth transitions.

Punching the pedal does nothing more than you trying to match up the airflow to the condition of the engine. These cars do not operate like past carbureted engines. There is no accelerator pump to squirt fuel.

And while humming around in those areas inspect that big convoluted air intake hose for rubbed in holes or looseness on the ends behind the Air Mass Meter.

In other words, vacuum leaks or uncontrolled amounts of air flow creating lean/rich fuel mixtures cause what you describe.

Phil