EPC lights up. Can reset and continue

In the middle of driving the EPC light comes on and it’s almost impossible to continue.  During the engine off then off usually clears it and the car runs normally.  OBDII shows Throttle Actuator Control P0638.  I’ve started clearing it as I drive which is not safe.

The main dealer wants £800 to replace the assembly but I can’t believe that it’s faulty as it works fine when I clear the error.  
Is there a way to permanently disable that error flag? Could it be a faulty contact?

any help would be appreciated!

Thanks