I have a very temperamental and annoying fault with my 1990 efi auto gearbox.When first started from cold,the gearbox will not change up even when iv changed the oil and filter in the box.To get the gearbox to change up i have to rev it in first gear up to about 3500 revs then it works as normal for the rest of the day until left overnight. the problem is no better even after changing the oil and filter in the gearbox.Any suggestions will be greatfully appreciated.

 

Sounds like you have a valve sticking in the valve body. Depending on your mileage, there may not anything you can do. If you wanted to try something, you might consider running a bottle of Lucas transmission additive thru your tranny, I know many people have had good results with it.
Mike