Static electricity, phonostage overload, bad cartridge or alignment.
I'd ground the spindle first. Get a conductive record brush. If you can, drill a small hole and attach a wire with a sheet metal screw. Attach the other end to ground. Clean you our records before and after play discharging the static. While the record is playing gently use the brush and see if the popping stops. If it does not then you have another problem.
Next would be the phono stage. Try another stage if you have access to one.
If it is static, your cartridge is very sensitive to it. I would consider a different one. Make sure it is loaded correctly. Perhaps your stage has too much gain for it. Turn it down if you can.
I'd ground the spindle first. Get a conductive record brush. If you can, drill a small hole and attach a wire with a sheet metal screw. Attach the other end to ground. Clean you our records before and after play discharging the static. While the record is playing gently use the brush and see if the popping stops. If it does not then you have another problem.
Next would be the phono stage. Try another stage if you have access to one.
If it is static, your cartridge is very sensitive to it. I would consider a different one. Make sure it is loaded correctly. Perhaps your stage has too much gain for it. Turn it down if you can.