peugeot 206 lx automatic gear box problem?

If you start and drive the car from cold the minute you drive up an incline the gearbox will either go immediately into 'snow and ice' mode or drop into it from which ever gear it is in, I have to stop the car turn off the engine and re-start again if the car is warm enough it will then proceed through the gears ok, also once I have warmed up the car (approx 10-15 mins left running on driveway, she grudgingly goes from 1st to 2nd gear after that she is fine (ish).
I took her to garage and put it on diagnostic machine they said she needed new sensors in gearbox, but to try a gearbox oilchange first as that might solve problem, had gearbox oil changed and car was fine for 2 weeks and then back to square one! The engine management light is on as well. Anyone any ideas what may be the problem is it sensors? It is an old car and have not got a lot of money any suggestions how I can get this fixed economically, as garage quoted between £ 400- £ 500.