How to make power supply to 434MHz receiver module more stable on RC car?
If your receiver shares the power supply with the motor, then it's going to drop at full power.  You could filter momentary burst of full power... but the longer the on time the harder it gets.  A separate power source would work.    

Regarding the receiver module, if it's that sensitive to the input power, then maybe add another battery and a three terminal voltage regulator.