Every now and then, even this morning i can hit a small pot hole or bump in the road going anywhere from 20-45mph and my truck bed will start jumping and shaking like crazy, i can see it bouncing around in the rear view mirror. Feels like im riding on a flat when this happens. It goes on for about 4-5 seconds but i can make it stop faster if i give the truck some gas. any idea what this could be? bad tires? seems like it worse on colder days but that could just be all in my head. Anyone had this problem?