I had a similar problem on my 91 945T. I couldn't quite isolate the final culprit. I thought it was the power stage, which is a solid state device located on the left front fender wall. Takes 10 minutes to replace and cost about 60-80 dollars. I also recall replacing the crankshaft position sensor or RPM counter, which sits on the bell housing right behind the engine. The wire for it comes up to a plug by the back of the cylinder head, if you follow the wire back down with your hand you'll come to it. One 10mm bolt holds it in and you'll need every extension on your ratchet wrench to reach it but it's no more than half hour to fix and 50 bux. Only do this with the engine cold otherwise you'll fry trying to reach it. These are easy fixes but as always your mileage may vary. I wouldn't rule out what others have pointed out. Check the diagnostic codes and good luck!

Have you cleaned the throttle body?

Dear Barry,

Good p.m. and may this find you well. I suspect the Idle Air Control Valve. It is located under the intake manifold on the driver's side of the engine (US models). The Valve has two hoses and one electrical connector. It is secured by two hex bolts.

With the ignition OFF, remove the electrical connector (on the end of the valve closest to the firewall. Loosen the two hose clamps, and remove the hoses.
Remove the two hex bolts.

Once you have the valve in your hand, you'll see at the end opposite that of the electrical connector, an opening. Flush that end of the valve with carb or choke cleaner. You'll see gray liquid coming out. That's dirt. It is causing the valve to stick.

You can use a Q-tip gently to brush away deposits. Make sure no cotton fibers are left in the valve!

If this does not help at all, the valve may need to be replaced.

Hope this helps.

Yours faithfully,

spook