It sounds like there is a ground for the chassis and a shield for cart leads, and these two are not connected together is that right? Or maybe they are connected together at the preamp ground screw?I guess what I'm getting at is that one of these ground/shields is a problem. It may be that the cart lead shield is bad or the chassis ground is bad or they are connected together when they shouldn't be, or they need to be connected together. It could also be that the lugs are not making good contact where they are screwed in.
This all just a guess on my part, but hopefully it will give you some idea of what needs to be looked at. Just try different things with the grounding/shielding to see what the results are.
Bill