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My 2000 Estema's overdrive only works now and then.
I have it switched on all the time. When cold and for first run of day it works fine until when I come to a complete stop, then it will not work again for the rest of the trip even when still selected on. Sometimes after a short stop ( like when getting milk from shop) it will then work fine until I come to a complete stop again and then it will not work again. I have had the switch replaced and the gearbox oil is clean and dirt free.
Frustrated. Any help appreciated.

This sounds like it could possibly be related to a sensor or electrical problem. The overdrive is linked to engine/transmission temperature and should only operate once the engine is up to normal operating temperature, so perhaps a faulty temperature sensor or wiring problem is the cause.
I'd suggest having it checked out by a competent auto-electrician or mechanic who understands how the system operates.