Do you have a fail safe installed?

I just installed one (Dynamite brand) and I started having the same problem (on the bench at least....haven't run the truck with it yet). I found that by turning the throttle HIGH ATV down just slightly the problem went away. In this cheapo transmitter there seems to be a slight detent (which I don't believe is supposed to be there) in the trigger action. It's like it is going to far.....past full throttle..... after I press past that detent or catch (whatever) then the throttle servo starts surging.

I don't know if this will help you but it just sounds like nearly the same thing.

Does your throttle servo surge when the engine is not running?...... or is it only when the engine is running?

Later......
Ron