Just an observation here
people are trying to help you----- and then you say that" you've weighed and you know you're not overweight"
well then why are you asking us if you don't want to listen to our suggestions?????
Sounds like your weight is not in the right areas
sounds like there could be something loose in your head( hitch or frame)
sounds like you could have a little bit of a load shifting on acceleration or braking
there's multiple possible issues.
I myself think the Andersen frame doesn't have the HEFT-- that us heavyweights have
I tow with the B&W
but that's just my opinion.