gearbox fault in the rain...

Hi all,just joined the site,hoping for some answers.I have a 1998 P38,drives well enough except when it goes through a puddle.It goes into lomp mode,gearbox fault shows up,after a couple of minutes or if I turn engine off and restart it the fault 9 times out of 10 goes no probs but just wondering what would be causing this

That sounds like you have a wiring fault around the gearbox and the water is creating a short. Look at the back of the gearbox and check the gray multi plug. Most of the ones i see are either broken or have been smashed off their bracket. Also check down the passenger side of the box at the gear position switch, water ingress could be a fault. If the fault clears right away after an ignition off I would say it is probably poor connections or chaffed wiring.
Separate all the plugs, check and clean with some sort penetrating oil or contact spray (WD40)

Could be the gear position switch, if the breather hose falls off water will come in. The fault can also be seen by checking the lights beside the stick and on the instruments, sometimes they show wrong gears, mine showed 1 on the stick light, R on the panel while it was actually in D. WHen I moved the stick, the lights jumped randomly.