Range Rover Gear change problems

[dookie]

I have this slight issue that the local RR dealer has not yet been able to repair. When you are driving for a long period and come to a halt, then when you drive off the car will not change up gear. I then stop the car and switch off the engine, hopfully to reset everything. When you try to start the car the engine will not turn over, although the ignition does come on. After approx 20mins the engine will eventually start, and when you drive off everything is OK.......???? Has anyone had any problems like this, and if so how do I repair it. The car is an 03' L322 with the 4.4 BMW engine. I have a feeling that this is a gear selection solenoid, question is where is this located in the car? It is becoming quite a pain as yesterday once the car was filled up with petrol it wouldn't start. After 20mins of waiting at the pump the car finally started, I felt a bit of a numpty just sitting there but hey I'm slowly getting used to it. Oh, The air bag light is on and will not go off. This was an issue last month along with the gear change problem. I have had the gearbox removed and replaced last week but this problem is still there. So after all this I'm looking for assistance from all the experts.