Check Engine light comes on only when it's cold out.

Hi I have a 1998 Chevrolet Cavalier with a 2.2 liter 4 cylinder engine and an automatic transmission. When it's quite cold out, the "Check Engine" light comes on on my dash board. I phoned up a guy I take my car to for repairs, and he told me to bring the car to his place the following Monday. Over the weekend, the Check Engine light went off, but I figure that the fault code would still be stored in the computer's memory, so I brought the car to the guy hoping he could figure out what WAS wrong. He hooked up his computer to the car and told me the fault code was for a problem with the "Evap Sensor". Since then, the Check Engine light has come one and gone off again more than once, and I've noticed that when the temperature is mild, it goes off. My mechanic told me that it could very well be that the cold temperatures may be causing the gas cap seals to be stiff and not seat properly on the end of the filler tube. He also said I shouldn't worry about it because it would probably disappear once the warmer weather comes in spring.