I think your problem sounds a bit vague to me, and I'm unsure what suggestions to give you.
Definitely try other Tonearm Cables, and/or if possible a different pre-amp-amp. The proximity of where you have your turntable in relation to the rest of your audio gear can have an effect.
It's hard to pinpoint when you say you go from having non-existant hum, to very bad.
You're saying that when touching nothing, one time the table plays fine, and another it will hum? Mark
Definitely try other Tonearm Cables, and/or if possible a different pre-amp-amp. The proximity of where you have your turntable in relation to the rest of your audio gear can have an effect.
It's hard to pinpoint when you say you go from having non-existant hum, to very bad.
You're saying that when touching nothing, one time the table plays fine, and another it will hum? Mark