Loss of power when raining

[Cactus draw]

Ive got a t25 vanagon lhd 2.1 1991 - drives great, always starts never any problems unless it rains ... Twice now in heavy rain it just looses any ability to accelerate and trundles along at 5 mph but will still start every time. Have ended up getting towed twice now to two different garages both of whom can't find the problem ... Then a day later when it's dry it's completely back to normal and driving fine and the garage says its too late to find out what's wrong. Any ideas?

[Mickyfin]

Have your heat exchangers rusted away, or been removed? If removed, have the piping been removed to airbox also, and blanked off? If not, this will let water into your airbox causing problems in the rain.

Welcome to the forums. Where are you located?