If it only happens when it is semi-warm, that is, after it has been shut off for 10-30 minutes, then that is normal for a 1991 and difficult to completely eliminate. If it happens when the car is cold, or when it is fully warm, then something is wrong.

I think that the problem is that this model's normal hot idle is fairly lean, so if it is not fully warm, or if your plugs are poor or old, or there is a small vacuum leak, or pretty much any other minor problem, the result is a warm idle stall.

Make sure you have no vacuum leaks, and check ignition wiring for corrosion. Use good spark plugs - I prefer Nippodenso 'U' plugs, but use Bosch or NGK when I cannot find the Nippodenso plugs. If you want to try the Bosch platinum 4-electrode plugs, shop around, prices vary widely among stores, and they are not cheap. I don't think that they are any better, you experience may be different.