Hmmm... that's a weird result... are you absolutely sure you have the test circuit wired up correctly? I just tried it with a good MOSFET and touching the tab does nothing. The tab is connected to Drain (same as Pin 2) so I can't see why touching it should do anything.
I'm thinking the MOSFET is bad but given that weird test result I'm not totally confident. I've emailed Ian, who designed that circuit, asking him to come look at this thread. Let's see what he has to say.
Unless you just happen to have an extra IRF640, in which case you should put it in the board and see what happens!
Look into it later when the dust is clearing off the crater.