i have a calli t 4x4 running 280bhp approx. when driving the car sometimes slips in to 2wd and the 4x4 light comes on, but it only happens when i go above 4000rpm in any gear and then my rev needle goes a little funny by flickering up and down, the 4x4 codes read 'no rpm signal' and 'brake light switch' this was tested with the engine running. i changed the cranckshaft sensor and there is no change would the brake switch make any difference. if the car does go into 2wd i switch the engine off and back on again whilst driving and its fine. also when i start the car up it sometimes starts in 2wd with the 4x4 light on but after 3 or 4 attempts its fine again.
Regarding the 280 BHP. Take very good care of the transfer box with particular attention to the tyres. You are running the transfer box outside its intended spec. Lots of people do and some get away with it and some don't. It is designed for 205bhp.
If you were to have a reconditioned txb from Cav16, you would not get the stardard 2 year unlimited mileage warranty without fitting the 'Sportsbox'. This doubles the drive to the rear wheels to cope with the additional power (max 300 bhp) and, by definition, reduces wheel spin on the front. It is the wheel spinning thats will principally deal the blow to the txb. The clue here is keep the wheel spinning to a minimum if you want to stand any chance at all.
I searched Cavweb for the 'Stubber' post and the one I found refers to the diagnostic plug and possible problems - and yes I have heard of this causing a problem not unlike the one you describe.
Standing in front of the car, with the bonnet up, and looking at the engine, the diag. plug is located on the top / back / right, as you look at it, of the bulkhead. It will be beside the horn of the standard theft alarm. It is a multiwired, multi pinned plug and is plugged into a storage socket. It is where a TECH 1 machine is plugged into to read fault codes from the various ECU's.
I believe what happens is that when it is parked there is a blade in the socket, which is at earth, and it plugs into one pin of the plug when it is parked. Sometimes the earth is bad or the blade is bent and does not locate.
i will have a better look tomorrow and check all the wires, but if the transfer box has had it would that cause the rev needle to go funny or is it because the revs go funny that the 4x4 slips into 2wd,
If memory serves the earth pin, which is part of the socket, goes to plug pin A. If you access to a Haynes you will be able to see it there. I would suggest you check/clean the earth pin and however it connects to ground to see if the earth can be improved. Then you need to check it seats properly into the plug.