Make sure any loose, long wires going every which way, are not near the Transmitter/Receiver pair as cross-talk would become an issue. Also be sure that the Transmitter/Receiver pair are not near any Electromagnetic sources, e.g. Motors, solenoids, other oscillators, crystals, etc.
Be sure to have the RF circuits being feed a regulated supply, voltage spikes, and voltage drops tend to mess with oscillators and their related circuitry. Another thing, add a ceramic capacitor of about 0.1uF and an electrolytic capacitor of about 10uF to the power supply rails of the RF circuits, this will filter out other spikes and help "smooth" the supply voltage.
You might also want to find some RF shielding for those units, as they look pretty exposed, but I recommend that you do the above advice before creating some RF shielding.