Too many engines on powered track can make this worse at start up.
You should make sure the wiring inside is not touching or going against the board set like in this post.
https://ogrforum.ogaugerr.com/...ce-needed-with-photo
If an engine takes off at full speed it maybe something different. I'm guessing here, but maybe it's locked into forward? Have you tried resetting it?
I will take another guess that the signal is weaker on the variable channel then on your fixed ones. So are there more engines parked on it? More track connected? Less power in? Have you swapped the channels to see if it clears up?
