Well, I finally got around, hopefully, to resolving this problem today, and it would appear the cause was not what I was expecting!
When I removed the encoder I heard something rattling inside, removed the cover and one of the small nuts which holds the pcb to the case had come adrift, when I looked closely at it I could see black deposits on the nut, the type I'd associate with arcing!
So I conclude the random altitude transmissions where a result of said nut shorting out on the pcb. Just hoping the shorting hasn't damaged the electronics
