try a ferrite choke on your coax or power/ground wires - you can get these at Radioshack for a couple bucks and they just snap over the wire - these are what the 'bulges' are in USB/computer cables and are used to cut down on interference from outside sources, like a filter of sorts
from RadioShack
Effective, easy-to-use RFI fighter.
Cut down on RFI (radio frequency interference) with this snap-together Ferrite Choke Core. Simply snap it over any coax cable, AC or audio cord.
Easy to use RF interference fighter
Just snap over cable or cord