Golf IV / Jetta IV :: 1999 - Erratic Throttle In Wet Conditions

I have a '99 Jetta 2.0, manual. In wet conditions (heavy slush/rain) the throttle will start to behave erratically, you can feel it against your right foot pulsing in and out. The feeling is almost similar to that when you put cruise control on and feel the car take over the throttle and pull it away from your foot and adjust its position. In fact, at first I thought it was a cruise control issue so I pulled the fuse - no better.

From deliberately driving through puddles in an attempt to isolate the issue, it seems that when something on the passenger side of the car gets wet the issue occurs, almost without fail if I drive through a big puddle on the passenger side. Once it starts, it continues for awhile (I guess until it dries). Even at a stop light, you can see the RPM's rise and fall and feel the throttle pulse without any input from the driver.

I have siliconed the ABS sensor on the passenger wheel, as it was the only exposed sensor I could see when I pulled the wheel off. My question: What sensor under the car could have an effect on throttle? Something is shorting or reading erroneous signals when wet and is causing the computer to think it needs to correct the throttle position. I'm at a loss as to what it could be and google searches have yielded little results.