@Gimbal: Play it further, you'll also get those slowdowns.
@FromWithin: I have a P4 2,6GHz and emulation is running at full speed with no frameskips
I verified the problem with the MAMEtesters board. They say those slowdowns are correct because the processors used on the original board were too slow causing these slowdons also on the original arcade game. The emulation is perfect, that's why those slowdowns are also emulated.
There are bootlegs of some games. The producers of those used faster processor on their boards, so these games run faster without slowing down. Seems as if I played on one of those when I was young
Anyway, the only way to speed up those game (there are a few games with this problem) you have to overclock the CPUs in emulation through a cheat.dat for MAME.
So it's really not my computer, it's the game itself.
Check the MAMEtesters forum and search for a topic by me if you're interested.