A couple questions...

If you park facing up hill and it rains, do you have the same problems?

Maybe it's not really the fact that you're facing downhill, but maybe when you are downhill, is the grill facing the wind so more rain is driving into the grill?

Either way, go under the hood and check ALL the electrical connections. Maybe even hit them with some contact cleaner and seal them up with dialectic grease. I used to have trouble starting my CJ in the rain until I put dialectic grease in both ends of all the plug wires and around the base of the distributor cap, haven't had any trouble since.