Automatic transmission fault on a 1.6 vectra arctic R reg?

Hello all,
My vectra more often than not starts with the fault light for the automatic transmission system on as usual but when i try to engage either Drive or reverse it comes back on and prevents me from moving forward with any kind of speed. Sometimes it will accept going into reverse then when i switch to drive it developes the fault again. It is usually fine when it has been run for a while, like if i use the car not too long after a decent run it is as if there is no problem at all.
I wonder if there is a dodgy sensor somewhere or anything alse i can do. The transmission fluid level is ok too.
any ideas would be greatly appreciated, thanks in advance.