1997 Vauxhall Vectra won't change gears after start-up

Car when warm seems to function perfectly fine, but sometimes, especially when cold, the automatic transmission doesn't work after start-up. Car seems to be stuck in third. On the display a check-engine message appears, specifying a problem with the automatic transmission. After driving around using manual gears and then restarting, the problem usually subsides.

I have read other web-posts which trace the problem to the gearbox selector switch, rather than the gearbox itself, and suggest that the problem can be fixed by cleaning and regreasing the switch. See for example:

http://sirbendy.co.uk/car-projects/vauxhall-automatics/fixing-the-vauxhall-flashing-s-of-doom/

Has anyone else experienced this problem and managed to solve it? Was it the gear selection switch, or something else? Thought it would be worth checking before trying to pull it apart and clean it...

Many thanks!
Muz