VL commodore 86' Gearbox problem?

[jetpilot83]

I'm no motor mechanic so could you guys help me out, I own a 1986 VL commodore (auto). Had it for almost 3 years, done just over 180k, nice clean car. I predict the problem is the gearbox, for the first five minutes of driving (unitll the motor is at operating temp, two indications on the temp gauge) the engine will remain in third gear no matter how I drive. I have tried starting in first, then into second then into drive, once the engine is hot it goes through the gears like new, its only when I first start driving that it wont go into the overdrive gear. Anyone suggest what the problem might be?