I have had this problem for a long time and can not determine what causes it. I have taken the car to a number of mechanics, just to pay a lot of money and get it back with the same problem. It is a dangerous problem as you must be aware pulling out in traffic that it can occur and be prepared to pull over quickly.

I have a 1991 940 turbo with 193K miles and it runs good, most of the time.

It has an intermittent problem where it dramatically hesitates. I pull over, turn the car off and back on and normally, not always, the problem will resolve itself while the car is running.
I have had the fuel regulator, distributor, knock sensor, fuel pump, fuel filter, and numerous hoses replaced. Cleaned the ICV, which helped a searching idle.

The car has no idle problems at any time.

When you need to accelerate, too bad. The car will bog down tremendously to the point that you normally have to pull over. It doesn't die and the idle is never affected. I have experienced various degrees of the problem. Once the car is running fine, the problem rarely reappears, as long as you don't turn the car off. Once the car is hot, it takes longer for the car to start, sometimes even turn it over for 30 seconds, stop and try again but it always starts.

It is really a shame because the car is in very good shape other than this problem but I will not let anyone drive it do to the danger.

During the problem, I have put the car in nuetral and "raced" the engine and had the same hesitation without the car in gear.