This has bugged me for a while. It has happened with Suse 10.0 and Kubuntu 6.10 and Kubuntu 7.04
I have used various drivers. I currently just run Kubuntu 7.04 and not sure which driver. 8.33.x I think from AMD/ATI.

So, I run Doom 3 in a window.
It runs alright but FPS slowly goes down. And I finally hit 1 FPS and it's extremely unplayable. Even going to the main menu of Doom 3, it renders at 1 FPS.

I have to close D3 and start it again, but most often, the same thing happens.

Anyone experienced this?

The other games I have are the free stuff and they are all fine.
For example, Chromium and Tux racer.

I am not sure about Kubuntu but for my Ubuntu 7.04 box I installed the 'ati accelerated graphics driver' and that solved all my ATI headaches. I am able to run Doom 3 with no problems.

Which drivers?
I have used the instructions on the ubuntu wiki to install the ATI prop drivers. I have used many different versions over the months.

It's a restricted driver called "ATI binary X.Org driver". You will have to install the restricted driver manager.

If Kubuntu has an Add/Remove Applications program like ubuntu does (under Applications -> Add/Remove) it can be installed that way (easiest). Otherwise you will have to apt a bunch of things.

HEre is an article that helped me: https://help.ubuntu.com/community/BinaryDriverHowto/ATI

Hope it helps.

I think we are talking about the same thing : ATI's proprietary driver.

Actually, it's not clear to me what the issue is. Sometimes, the FPS picks up. I think it has to do with what direction I am looking at.
This could possible be bad optimizations in Doom3 CPU code (math and logic) or the graphics drivers.
I was wondering if I'm alone in this? Is there something I can do?

Do you meet the system requirements for Doom3? Sounds like stuttering from not enough RAM. If you do meet the requirements then sorry I am of no help. But I would try another game that is graphic intense just to see if you are getting similar problems.

Next week ATI is finally releasing their new driver built from the ground up..
Phoronix has been releasing articles about you can read'em here.

Looks like ATI users will finally have gaming performance in Linux that only nVidia offered before.
1 more month for AIGLX support though, this release finally offers x2xxx series and better x1xxx
card support.

Sure. Athlon 2800+, 1.5GB RAM, even at low settings like 640x 480 and lowest rendering settings it doesn't help.
Under WinXP, it runs fine (or at least way better)
Before, I had 512MB RAM and it was the same deal : ok in Win XP but Linux ...
Then I added 1GB stick.