IMO you are now among the many of the too little (TV) or too much (TT) crowd with probably too little TW and possibly using a hitch not quite up to the task you are using it for. The last is fairly subjective and open to debate. I also agree with another poster that was questioning just how accurate your weight "GUESSTIMATES" might be and my money would be your weights are most likely on the low side. To add further is the fact that all these highly optimistic towing capabilities put out by the manufacturers are based on a 60sq ft frontal area which is usually exceeded by amost 50% with most full profile trailers and you have even added in some significant winds th
at make that "HUGE BOX" even more susceptible to not corporating.
Correcting the most significant issue which is not enough TW will probably drive you further over some other rating such as AWRs or TV GVWR which the only good solution is a more capable TV like a 3/4 or even 1T. In that regard I'm sort of in the camp that if you're going to go to a 3/4 then why not just go the full 1T route even if a 3/4T would satisify your current needs.
To get back on the right track I would recommend you getting a tongue scale to make sure you actually know what your real tongue wt. is and have the capability to adjust it without making a trip to a set of scales everytime you load differently. A second step or even the first one is to get to a set of scales and get some real numbers so you have some actual "FACTS" to work with instead of what are probably overly optimistic and low weight guesses then come back with all those numbers and we might be able to give you some meaningful help. Until then anything here are mostly just guesses on our part with a healthly or more accurately unhealthly doses of optimism.
Larry