Car dies when damp and at highway speed

My sisters car only seems to do this when on the highway and is damp. She then gets out and fiddles with the gas cap and it starts. But while it is dying the engine losing power light comes on first then the engine has no power light.I ordered a new gas cap and the parts around it because the are broke and loose. My first thought was that moisture was getting in through the wore out gas cap. We also checked the battery to make sure it was in good shape. Could it be a sensor getting damp? Which one? Any help or questions? Thanks