Nasty hum on my new Audio Technica P120...help!

[BogoEN]

Ok so here's the deal;

I recently bought an ATP120 off of Amazon. I assembled it, started it up with the stock cartridge, and it sounded fine. HOWEVER, I invested some extra $ for the AT 440MLA cartridge. Tough to mount on the headshell, but I did it, making sure that the same set up was used as with the stock cartridge (plastic covers, etc). Well, I put the headshell back on the cartridge and there is this wicked hum. Not TOO noticeable, but enough if you plan to rip some vinyl onto the comp. After talking things over with my father on the phone, trying to make a ground line connection (which the table doesn't come with), no results. The only real result was when I would touch the tone arm with the ground line, which tells me there is a ground loop in the tone arm.. The worst of it is that I have read of this problem before with this table and AT just sends another one out, usually with the same problems. Can anyone help me?