I have a similar problem with my X-Pole (45mm TG - not sure what model, husband bought it for me over a year ago).
It doesn't really thunk, but it does seem to have one spot in its rotation that it gets hung up on, almost like the rotation is lopsided somehow. When you're actually up there spinning on it, you don't really feel it, and it does make for a slower spin (which I don't mind so much). But if you're just standing next to it and turning it with your hand, you can feel it "catch" almost?
I've been trying to figure this out forever. At first I thought it was because I had it set up on carpet and the carpet was uneven, but I'm in a new house and have wood floors, and it still does it. But not *every* time, which is the weird thing. Every once in a while I set it up and it spins smoothly, but most of the time, it has that catch.
It doesn't look bowed to me, nor does the level indicate that it is (though I will have my husband take a look too). I'd be surprised if bowing was the problem, because it's been doing this pretty much since I got it.
I suppose it could be overtightened - after having the darn thing come loose a few times while I was on it (no injuries), I'm a little paranoid about having it tight enough to prevent that.
I was messing with it last night and noticed this time it wasn't just getting caught, but groaning as it rotated, and the groaning seemed to come from the dome, so I'm going to take it all apart tonight and get a closer look.