I think you have not been quick enough with the set up and you have trimmed it to stop the car from shooting off. That's why it's ok after a few seconds, as the TX links with the RX and trims it back. (This is an assumption)
Here's what I would do: -
1 - Support car - Wheels off deck
2 - Go to return to factory settings on DX3R (If you don't want to lose all the memory settings, just return everything to Zero as mentioned above)
3 - Bind the DX3R to car. Turn everything off
4 - Turn car on whilst holding button on GTB switch. When Light turns red, be quick - release button straight away, then go to full throttle until solid green, then trigger all the way forward for full brakes, GTB flashes green then neutral and you should get solid red.
Turn car off, turn transmitter off. Turn car on. It should be as calm as baby on smack.
Hope it helps.
