had the same problem when I first brought my Nextar to the field twitching with the engine running or not antenna was run through the fuse,but the antenna tube is run between the pushrods they are wire and was probably the source of interferance,it was the most obvious scenario for the interference problem.

easy solution done at the field, I opened a hole in the fuse,in my case the top, but the bottom would have done just as well, took a servo arm for a keeper and a small piece of fuel line pulled through the fuse so the antenna doesn't rub,put a T pin in the tail and used a rubber band to hold the antenna wire,problem solved.

if exposing the antenna to the outside of the fuse doesn't fix the problem then you might need to send it in for a check up.