The first knee jerk response I have is to get your dolly tires balanced.
Then I would try to tighten the connection in the sockets themselves. Can you get a pair of needle nose plyers into the socket and squeeze the contacts a little tighter?
Last I would try to secure the wire loom as close to the plug as possible.
Mike
2006 Chevy 2500 4X4 crew cab Gasser
2013 Dutchmen Denali 311 BHS (Love the trailer HATE the manufacture!)