I had a 20' 14k bumper pull trailer doing that to me and it ended being the tires. They would develop a flat spot from sitting that would take 50+ miles to straighten out. I would load it a little heavier on the tongue and that made it tolerable. I ended up replacing the tires, worn and weathered, and all the bs went away. If I'd of known that I'd of replaced them years ago.
The Kaufman guy may have been trying to tell you they put on junk tires without admitting to it.
Have the axle alignment checked. If that's ok replace or borrow a set of tires to see it that's it.
Measure the axle placement on the trailer. 60% of trailer in front of axles and 40% behind seems to be the best for bumper pull trailers.
I had to back my 5103 Deer and 6' shredder on that trailer to get it to balance properly. If I pulled it on I would have too much tongue weight by the time I had the shredder most of the way on.
I went to a 14k 20+5' deck over gooseneck and will never go back to a bumper pull.