Sounds like a couple of possibilities. First is low oil pressure, which can be checked, I believe, from a fitting on the outside of the case. I don't know the spec, but it ought to be at least 65-75 psi. Another possibility is a worn out clutch pack or a sticking piston/accumulator for the front band. In either case, don't sit on this one. Get it fixed before a major transmission casualty occurs. Other than keeping the case cool, I have no clue as to why rain should affect the problem.. Get it looked at ASAP. Good Luck..
1987 BMW 325
[This message has been edited by jeffsr (edited 04-06-2000).]