What do cartridge problems sound like?

I've just had my tubed phono pre serviced trying to track down the source of loud pops and snaps (not surface noise), and intermittent gritty, fuzzy, distorted sound. Strangely, these symptoms are less prominent after the pre was serviced, but have not disappeared completely: some sides are unlistenable all the way through, some play flawlessly and beautifully, while others are some combination of both. Initially, the distorted sound would show up about 15 - 20 minutes into playback after a 30 minute warmup, and stay for good. I am now wondering if the problem is somewhere in the tonearm or cart or some of that wiring, which is not easily accessed and fixed by me. I've already eliminated all the other possibilites: cables, preamp, power amp, etc. All other source components work fine with none of the above issues. Music Hall 5.1se, Goldring stock 2200 cart that came pre installed on the table, and Hagerman Cornet 2 phono pre, all of which performed perfectly up until a few weeks ago. FWIW, the phono pre is a factory, not kit built unit with Multicap capacitors. The tech replaced a loose rectifier receptacle (where the rectifier tube plugs in) and ran it without problems in his system for several lp sides. Any thoughts or suggestions are greatly appreciated.

Have you cleaned the stylus? Check the tracking force? Checked the antiskate?

Don't make us do all the work here.
It could be a loose connection somewhere. Also, a tube can have an intermittent problem. You could try tapping each tube lightly with something like an eraser, or other item that won't break the glass to see if that causes it to change. A part starting to fail could cause this too.
If this just started to happen, with the approach of winter and lower humidity, it could be a fairly extreme case of static electricity. A good room humidifier is a wise purchase in any event (unless you're in the tropics).