You can get more noise "filtering" by making your pre-amble more unique. Right now you are using 9, which is 0b00001001. So anytime the receiver puts this out the PIC will display the next character. You can see (in particular if you look at David's plots) that you can almost guarantee the receiver will output this at any particular time. That is, a noisy receiver will output ones randomly.

So if you make your pre-amble longer and more "unique" you gain some advantage over the noisy output of the receiver.

Say:
Code:
SEROUT TransmitterPIN,T1200,[Synk,Synk,Synk,Synk,Synk,Synk,42590,Counter]
and:
Code:
SERIN ReciverPIN,T1200,[42590],Counter
This of course is in addition to all that has been said here. The RSSI output is a nifty way to create a "listen to me" or "wake-up" signal, and FM-based systems (e.g. FSK) modules have an inherent advantage over noise. However you can get the AM-based (e.g. ASK) modules to work ok if you understand the limitations and caveats.