Car runs rough for 5 minutes after filling gas tank.

I have a 1996 Volvo 960 wagon. Every time that I get gas, for about 5 minutes afterward, the engine runs as if the fuel mixture is off, the engine misses, runs roughly and feels like it might die when I am at a light or the engine is idling. My mechanic has not been able to figure out what it is. I believe it is related to the pressure in the gas tank because after 5 or 10 minutes of driving the problem disappears and I don't have it again until I get gas and the gas cap is removed.