Since the initial tests shows how jittery the motors are. It is time to take a look and see what is going on.
To test whether or not it is the fault of the code, I ran the RC car as it was connected to the pwm generating atttiny85.
A voltmeter was attached to the input pin of the receiving pin and ground and the measurement was the frequency.
This was also done when the modules were used (not connected).
Here are the results:
However when the modules get involved that is when the trouble begins.
The frequency jumps all over the place, therefore I will assume that some sort of unity gain buffer or amplifier will be needed.
I also believe that using variable duty cycle as a signal protocol/encoder is not the best option. It may be beneficial next time to try to use a different form of encoding/signal protocol like UART, USART, ICP, SPI, Manchester and etc.