'97 Ranger stalls when idling ONLY after overnight rainstorm

I have an odd problem with my '97 Ranger (XLT, 4cyl 2.3L, Automatic, 99,500 miles) I'm still trying to pinpoint. When we have a heavy rainstorm overnight, the next morning the truck will start right up but whenever I slow down/stop at intersections the truck may begin running rough and/or stall out. When this happens I've resorted to throwing the transmission into neutral and braking towards the stop while holding my foot on the gas to keep the engine running. If the truck does stall out, I can get it to start right back up as long as I keep the RPMs up with the gas petal. Once I get onto the highway the engine runs fine, and the engine will run at idle just fine by the time I get to work (about 15 mins of highway driving). Furthermore I've never had this problem when it rains heavily during the day and I drive home. I've popped the hood one morning when this problem was occuring and the engine compartment looked dry- I didn't see any condensation or water dripping onto any wires anywhere even near the firewall.

Some further elimination points: Headlights and wipers are off, and it's a pretty new alternator. The fuel filter is relatively new. The car can be in park and still have this issue. The check engine light hasn't come on.

My current suspicion is that water is somehow getting into the gas tank. I'm going to try dry gas to see if that helps, but since the problem is semi-intermittent I wanted to see if anyone else has experienced this problem. I'm concerned if I take it into a shop they'll start out on a costly elimination spree including plugs/wires, O2 sensors, etc, etc.

Thanks!