1995 E300 pressure in fuel tank

[packerfan]

My 1995 E300 has started building air pressure in the fuel tank whenever the engine is running. The engine has stalled a few times on me when I stop at a stop sign. I opened the fuel cap on the tank a bust of air rushed out.I have since been driving the car with the fuel cap not screwed down tight and the stalling problem has stopped.
What would cause this pressure to build in the tank?

Thanks for your help.