My 535 idles very rough, like a galloping horse at lower rmps when it is raining or with very wet snow. I realized there is not a plastic engine cover to protect the underside of the car as many years back I trash it on a parking block. Could that be the culprit? Is something getting wet? If I "floor it" the car will get up and get it on.

I did the standard things replaced plugs, dist cap/rotor, idle control valve, vacuum leak check, replaced fuel pump(for other reasons) and fuel filter.

Again this only happens in high moisture conditions.

Any suggestions?