I once had the zPhono, Music Hall MMF2, and the Goldring cartridge that comes with it, so I know that set of equipment.
I had problems with used classical records. They sounded thin and the pops and clicks were completely overpowering the underlying music. I wouldn't refer to it as muddy, but the signal to noise ratio was horrible. It sounded OK on pop music LP's, but nothing to write home about.
Just to be complete, have you properly adjusted the VTA and tracking force? Does this happen to all records or just a few? What condition are the records in? Is there significant warping, for instance?
To be honest, overall I just wasn't happy with that setup. Now that I have a KAB/Technics SL-1200 and a Denon DL-103 cartridge, I am MUCH happier with the result. All of the records are at the very least listenable, and some sound extremely good. I'm not sure if the turntable or the cartridge is to thank, or likely the interaction between them.
I found the zPhono to be a pretty decent phono preamp for the price.
If you have anyone near you who is good at setting up turntables, you might enlist their help to make sure you're not missing something obvious. But consider the possibility that you might want to save up for gear that is a notch above your current MH turntable and Goldring cartridge.