Likely causes:
  • Voltage spikes/noise from relays and/or overcurrent causes Brownout on PIC
  • Voltage spikes/noise from relays induces garbage on receiver
As Robert suggested; interconnect diagram / schematic will help.

Things to consider:
  • How are the relays connected?
  • How much current do the relays require from the PIC's I/O?
  • Are the relays isolated from the PIC? How about from the RF Receiver?
  • How many times do you receive the send/receive the control commands?
  • Have you verified the TX PIC is still transmitting information and the RX PIC is not responding (this probably is the case, bust just to be safe)?
Just somethings to consider.