I agree with alexh, you need to tweak your WinUAE settings and/or tune your Windows install.
No I don't. I've got a MiniMig :-D
Something to keep in mind -- there are plenty of PC demos which run 100% smooth,
I've seen a lot of them and although there are some really beautiful ones, they suffer from the same issues on all Windows systems I've seen. It's not the programmers' fault. it is simply Windows.
and plenty of super-picky PC FPS gamers who are NOT complaining about hiccups.
First of all, most of those gamers are not used to the way things were with Amiga and even if they were, they long forgot all about it and only compare things to other PC games.
I've had numerous discussions with some of those gamers. They say something runs smooth, while I see hickups. Also, lookup any guide on modern games programming. The thing they teach these days is to measure the time that elapsed between your last redraw and the current and adapt your motion and actions to that time.
Why? Because timing is not constant and if you want to keep your game running as smoothly as possible, you need to work around that.