Gas fill problems

[anziani]

Checked the archives but couldn't nail my problem. I was on a long trip and gassed up. The pump shut off. I put on the gas cap and was talking to the guy at the next pump when suddenly I smelled gas. I noticed a pool of gas under the trunk on the pavement. I took the gas cap off and "whoosh" a 1/2 cup of gas sprayed out. The same thing happened later that day at the next fill up. It acts like pressure builds AFTER the cap is replaced on the fill up. The car is my '93 300CE. Any ideas?
Anziani
'93 300CE 192K
'95 E420 122K