Manfred,
The description of your problem sounds like the O2 sensor has failed. When my 90 3.6 auto failed at 111k miles the fuel mileage dropped in half, the engine shook at all speeds, the idle would fluctuate 400+ rpm, and would vent black smoke from the exhaust. A key clue is that the car feels fine at full throttle. The test is to unplug the sensor and if it runs fine you have found your problem.

Keith
1990 V8q #527