Transmission Very Jerky

[V8 Muscle]

On my 1999 GMC dually 4x4 im having issues with my transmission. It doesnt happen all the time, some days its perfectly smooth through gear changes then other days its really jerky and bangs into gear. I have also noticed on some days that it sticks in 2nd when in drive and wont change up, even if I move the gear shifter down to 3rd and 2nd it doesnt alter anything. The transmission oil is on its right level and there is no leaks. This is driving me crazy as I cant ask no one out here in the UK as they dont have a clue about American trucks. If anyone has any ideas of what I can look at or if they know what the problem is then can they advise me of somewhere to get the parts from. Thanks guys.

[V8 Muscle]

Can anyone point me in the direction of anyone who maight have any knowledge on this problem. Maybe someone that works on GMC transmissions. I have no one in the UK to go to.

[Fortune50]

Try to locate a Chevy dealer, a GMC truck is pretty much exactly the same. Other than that, call up some service centers that work on transmissions to find out what they might know, perhaps they can direct you to a place that can help you...