It's the sort of thing where I'd normally say it must be some kind of grounding issue, but Strat pickup polepieces aren't grounded anyway.
The only thing I can think of - and I'm not sure if it would make any difference - is that you may have swapped round the ground and hot wires on each pickup? It might be worth swapping round the back and white (or black and yellow, or whatever) wires, just to see if it has any effect. Of course you'd need to do it on all three so they're still in phase.
One other thing I read somewhere (the Kinman website?) to do with a similar issue: if you paint some clear lacquer on the exposed tips of the polepieces, it insulates them so you're not grounding the polepiece when you touch it.
But I'm not sure any of that will help.... :?
