I’m pulling a 15 foot light weight aluminum boat on a steel trailer with an astro van and reese hitch. When I hit 65 mph the steering wheel shakes up and down severely for 5 seconds. Suddenly the steering wheel stops shaking and the reese hitch begins rattling and shaking for 5 seconds. Then it goes back to the steering wheel shaking. This cycle repeats itself over and over again. Does anyone have any ideas as to why this is happening?