Did you check the cartridge connections to make sure they are secure and correctly installed? Then check the ground on your tonearm cable to make sure the ground is not broken.
To me it sounds like something in the arm is causing the problem. A broken pin or wire perhaps.
I would also check underneat the turntable to make sure the wiring is good. The last table I had like that I rewired the tonearm with better interconnect and it was a huge improvement. You also need to look at areas underneath that need lubricating. Is the grounding scheme on the TT okay? That's where I would start. I don't feel it's your preamp.
To me it sounds like something in the arm is causing the problem. A broken pin or wire perhaps.
I would also check underneat the turntable to make sure the wiring is good. The last table I had like that I rewired the tonearm with better interconnect and it was a huge improvement. You also need to look at areas underneath that need lubricating. Is the grounding scheme on the TT okay? That's where I would start. I don't feel it's your preamp.