1998 Volvo XC70 Engine dies or surges

At random, unpredictable intervals the car will die. Most typically it occurs when you stop the car with the engine idling such as at a stop light or sign, or in stop and go traffic. It has also died while maintaining speed on the expressway and the engine RPMs will fluctuate wildly and then the engine dies. Hitting the gas doesn't effect the RPMS but can sometimes be kept running if I shift into neutral and play with the gas pedal. All of this occurs randomly---sometimes when the engine is cold, sometimes after it's warmed up for 30 minutes or more. Some days it runs fine without incident, while other days chronic problems. Other times the car will have problems for a while and then run fine for a long period of time. Weather doesn't seem to be a factor and, of course, none of this happens when the mechanic has the car.

The fuel filter and fuel pump relay have been replaced, and the mechanic has been driving the car for 4 or 5 days now with a fuel pump pressure gauge, but none of the symptoms show them selves.

Help!