Cap moves around during driving

Hey guys, I have a snugtop camper shell (used, no idea of the model) that's moves around while driving. Like it wiggles and shakes but doesn't actually move from it's position on the bed. The clamps always seem to be staying tight. I've tried two different types of clamps but haven't had any success. The drivers side appears to move a fair amount while the passenger side doesn't. The clamps are always very tight. When you grab the topper and shake it the whole truck moves. It seems very secure but when i hit bumps I can see it wiggling. The original snug top clamps are rusted out and gone. Any thoughts?

 

Are you sure it's not actually the truck bed moving? The frame will flex and the bed will move around quite a lot. When you bolt the bed down to the frame, it will not always align perfectly. Measure the gaps between the bed and the cab on both sides. Also look down the sides of the truck and see if the bed has shifted left or right. It could also be your cab physically moving. Have you ever replaced the body mounts? Your truck is over 20 years old, rubber doesn't really last that long.

 

I've also seen it in the mirror while I was driving. I'm guessing it's just the bed flexing while the truck goes over bumps.