94 F150 with 178k on her, runs fine when dry…when it rains or is really humid the transmission starts slipping like it cant catch a gear. Has done this since I bought it with 50k on it. It has the 300 straight 6 automatic no a/c

You’re sure it’s the transmission slipping and not the engine misfiring?

At times it feels like it’s misfiring (shuttering). The truck jerks like it not firing on all plugs. But just yesterday in a downpour the engine raced smooth, the tach went to 3500 rpms and the tranny provided no power. Then it was like I was stuck in first gear. Then it shuttered off and on until I got to work. Now I did notice while under normal weather if I let my foot off the gas and hit that sweet spot between applying gas and not having to give it gas I sometimes get that shutter. It goes away when I go up a hill or apply more gas. Any ideas??? Thanks…

I’d try new quality spark plug wires (and new spark plugs too.)

This is an electronically controlled transmission. It sounds like water is getting into the circuits and causing mayhem. This really should have been addressed 128K miles ago. With the years and miles of accumulated damage happening, the problem may now require an overhaul at this point.