Well Wullie,
To give any kind of useful diagnosis I’d really need a bit more information.
Assuming the speed controller work okay when you were closer to the model than 23 paces, but goes “wonky” when you are greater then 23 paces away the problem is clearly with the transmitter or receiver.
You seem to suggest that all worker fine at some time, but not when you took the model to the flying field.
What caused you to swap the speed controller over?
Do all the servos work properly at a distance of 23 paces?
I think the first possibility is that the Tx or Rx batteries might not be holding their voltages.
1/ Check to ensure that both batteries have the correct voltage. (sometimes old batteries discharge themselves quite quickly.)
2/ Check that the receiver antenna has not moved and lodged itself in a position where it’s being shielded my something metallic, or close to carbon fibre material.
3/ Check that the connector from the speed controller to the rx is fully inserted and that the lead is not cracked or showing signs of fraying.
If the power system worked at one point, but isn’t working now then clearly something has changed. So it’s a case of find What! :!:
Good Luck
Keith