Don't know if you got your problem fixed, but I am having the same issues. In April this year I had to have the gearbox fully reconditioned (cost £3200) by TES Transmissions in Westbury, this was after a vehicle servive by TH White of Swindon which cost me £1900 as I only found out then that the car had not been serviced for 2 years! I purchased the car in March 2010.
Anyway, since having the engine sorted and gearbox done, on a return trip from Devon I got a 'Check coolant level' message (which was OK), then about 6 miles from home the car would not come out of 3rd gear, with no error messages! I then found out about the gearbox oil overheat and this 'safe' mode.
I got the gearbox people to check the car and they found no errors in the box, but suggested a radiator change as the VIN number of the car is in a range where there is a known problem where the bottom 6 or 8 rows of the radiator collapse internally, which is where the fluid takeoff for the Gearbox Oil cooler is.
So, radiator changed at a cost of £700 (incl labour and VAT), and on a shortish run to Salisbury, on the way back the sodding thing did it again!
Its since been back to TH White for a diagnostic check, again no errors, so the suggestion from them is to run the car to see if it happens more frequently to try to get an error code.
Next big(ish) thing to replace is the Gearbox Oil cooler which is going to cost around £500 with labour - But I dont want to spend that much if it isn't that!!!