434 RF Transmitter and Receiver for distance measurement problem
It is very likely you have an AGC problem. Try to slow down the measurement. The sender shall send data not often than once per second. 

This idea is not fully perfect for doing a distance measurement. It measures incoming power strength - affected by walls, reflection, AGC of the receiver, and a lot more. If you have a scope, measure the waveform on the pin 3 of the module (linear out). It is not a constant voltage, I am afraid, so you need some sort of averaging too.

A tricky way to get along even if you have an AGC is to measure how long the AGC needs to settle. The AGC works as follows: it detects the signal, and within some time (say, 2ms) it will change the amplification so that the signal will look having the full amplitude. 


compare on a two channel scope the signal pulse driving the transmitter
and the signal on the pin 3 of the receiver module


If you are lucky, you'll see that actually pin3 or pin2 output is delayed a bit compared to the transmitter driving pulse. The delay depends on the power of the incoming signal to the receiver, which depends on the distance.

I was able to make an acceptable distance sensor from a standard AGC enabled IR receiver using the principles above.