Bugger
Replaced the RA motor. Interference is still there. I had a closer look at the board though and one of the capacitors seems to be bulging on top and also appears to have a hairline crack on top too whereas the rest of them are flat. I've never seen one like that before so I'm guessing it has developed a fault.
A quick trip to JayCar tomorrow and I still might make it out in the field this weekend !!
Incidentally, I got one of the hi-torque motors from AusXMods (
http://www.ausxmods.com.au) for the princely sum of $20 so not too worried about replacing a perfectly good motor. The new one sounds so much better too so I might replace the DEC one too !!
Watch this space !!!!!