I've had a similar problem with mine in the past. Previously, after cleaning the mating surfaces thoroughly and drying them, I would smear some RTV on the housing surface and press the gasket onto that, and then let it cure for awhile. This would (usually) hold the T-stat in place while placing it up against the intake and threading in your bolts.
However, a better, quicker, and faster method involves simply putting a drop of Super Glue on either end of the T-stat housing's recess and setting the T-stat in there, so it'll stay put. As mentioned above, the T-stat has a tendency to slip out of place sometimes while you're wiggling things into position to put in your bolts, and that lip of the T-stat can get caught in between the mating surfaces, thus causing a leak. It's happened to me, before.
Also, making sure that both the intake and housing surfaces are truly flat is a good idea, as well.
My problem with the Pervertible, lately, has been that the T-stat housing bolts somehow keep losing torque and loosening themselves, and I keep having to re-tighten them to stop the thing from leaking. I'm gonna have to do it a third time this weekend.