gas cap pressure

I have a 89 cabriolet w/1.8 and when I take off the gas cap it is under alot of pressure, and after adding fuel the car stalls at idle for a while then clears up. Should there be any pressure at the cap? Is the cap bad, or should I look elsewhere. Another problem is if let the car idle for 5-10 mins then take off the car boggs like it doesn't want to take the gas. Then it also clears up after driving for awhile. Thanks TOM