I recently had a 1/3 ground force lowering kit added to my truck. Now when I hit a heavy bump I hear a thump like something is hitting something. Please assist. What is it and how do I fix it.

 

you are prolly bottoming out.

Check out your rear bump stops. Try cutting them or getting shorter bumpstocks. Usually it takes a 4-5" drop to bottom out bad.

You can also C-notch ur frame to clear the stops...

 

Then you'll be smacking your swaybar on the frame...

 

Is there any cure, besides cutting, should I buy another kit?

 

Go buy some carpenter's chalk. Sprinkle it on one surface of any suspected contact points. Find a really rough road near your garage and try to force contact. Then get back underneath and see if any chalk has been transferred.

I have also used grease and masking tape with the same effect.