I've bought Painkiller just today and I am having this weird issue - the game seems to run at a very inconsistent speed. For example, when the protagonist is simply running forward he seems to accelerate and then slow down at random intervals. It's the same with enemies, they often start moving at some Flash Gordon, crazy speed and then go back to their normal pace.
I am not sure if it's indeed an issue or if it's something that's just a feature in the game itself (though I find it hard to believe that something this weird would be intended). It's not really THAT gamebreaking but my problem is that this irregular pace is making me sick in real life, as in, nausea and that stuff (ye, that and the stupid head bobbing, but that's easy to disable) and after a couple of minutes I find it impossible to play. It has never happened to me so far, even in Mirror's Edge which is supposed to be really potent in making people puke :P.
Anyway, sorry for the rant, does anyway know is this intended and if it's not, how to counter it?
PS. I am running it on a laptop, Asus F3S, Core2Duo 2,0GHz, GF8600M, 2GB RAM and a fresh Win XP Pro + SP3 install.

As its a laptop make sure its well ventilated so its not overheating. Next check your power options, make sure its running in the desktop powersetting and not the energy saving settings. Painkiller is still a bit of a resource hog despite its age.
You could also check your page file with 2gb ram you probably want 3.5-4gb of pagefile.
I also don't know if you run anything like cool and quiet or anything that slows cpu use down, if so it may be a good idea to disable it while playing painkiller.
If you need any instructions on how to check or change any of this feel free to ask.

Whoa, that was fast.
It's not overheating and it's set to High Performance in Windows Power Settings (also, it's plugged in so, I think, it should set itself to max anyway).
No cool and quiet.
I am not sure about the page file so if you could kindly explain a bit about that, it would be nice.
Just a short note, though. It's not the case of framerate going down like in any other kind of game. It's quite the opposite, it's speeding up, that's what amazes me. I don't have Fraps installed to check the exact FPS but I am quite positive that it never goes below 30. The point is, there are moments where it runs so ridiculously fast that it looks like one of those old DOS games ran on a modern computer. And because it seems to last only a second or even a split of second, it is really, REALLY annoying.

My Computer, click Properties,Click Advanced tab then click Settings in the Performance section, Click the Advanced tab and then click Change under the Virtual Memory section. I would set it to 3072mb at least.
You could also disable the second core while running painkiller. To do this start painkiller up and alt tab out. Then open up windows task manager with ctrl+alt+del. Click on the Processed tab and find painkiller. Right click on the game or application and select Set Affinity. You will see CPU0 and CPU1 checked, uncheck cpu1 so its just running on the single primary core and click ok.

Your description very much resembles a problem some engines have with dual-core processor timings, but I don't think Painkiller (or, the PAIN engine?) has this problem. If Ralackk's "disable the second core" solution works, that's probably it, in which case the AMD Dual-Core Optimizer (which should work with Intel chips also, though that's based on what I've read, NOT experience) will solve your troubles permanently.
But I doubt that's the problem you're having, as Painkiller doesn't suffer from this AFAIK.

Did You try to turn on Vsync? Sometimes it helps with other games, when they work too fast. ;)

I've had this problem before. Set Painkiller's processor affinity to just one core; this is a common issue with the game and modern processors.

I have this problem too, and I'm running it on a single-core processor. Enabling or disabling v-sync doesn't help either. Very weird.

What video cards/driver versions do you guys have? I have the retail copy and haven't experienced any glitches like this.
Quad Core 6600
Win XP SP3
nVidia 8800 GTS 512 (forget driver version)
Do any of you experience sound glitching when the speedup happens?

It's a known issue with Painkiller and laptop graphics chipsets / dual-core CPUs / both. I had the same issue on my laptop, ATI chipset.
Installing the third-party Omega Drivers instead of those released by ATI and setting the affinity to run on a single core only is what fixed it for me.

Sorry I didn't see your post earlier. To answer the question, the Omega Drivers aren't compiled from custom source code. They're more of a custom re-release of the official drivers, in a more lightweight package and featuring certain performance tweaks. As such, they should work just as good as current ATI drivers.
However, the Omega Drivers have not been updated since 2008 now, and they do not function under Vista. But Laptop drivers aren't updated that often either. I can't find a newer version than 2006 for my ATI x1600 graphics card.
I've had no noticeable difference using the Omega Drivers to run other games, including such taxing titles as Crysis, Neverwinter Nights 2, The Witcher, etc.
HD2600 should be supported, but the 3000 and 4000 series are not currently supported by Omega Drivers.

Thanks for the reply, sadly I am running vista, oh well.

official solution:
http://www.gog.com/en/support/painkiller/Fluctuating_game_speed_