Game freezing in irregular intervalls

[Myxir]

Since the latest Update I suffer frequent freezes of Regnum. After an uncertain time of gameplay the game freezes shortly for one second. This lasts around a minute and then the game runs fine again. The game freezes every second or third second during this period which makes playing impossible.
Also I got the following issue several times today. I saw a player moving towards me. Well... I did not see the player, I only saw his turban and his staff. I was not able to choose him as my target. However, in Windowed mode the screen shot function does not work anymore.
I wanted to attach a screenshot here but it was not saved in the screenshot folder.
I use 169.12 Nvidia driver, released in February, with a GeForce8600GTS.
No, I won't downgrade my driver, since older drivers show the freezing problem, too. I lock out a system problem, since Regnum is the only Application with that problem. UT2004 is running fine. By the way, the new nvidia drivers improved the performance for 8xxx series a lot. In Regnum I get ~20 FPS more than before.
Well. I am still using Ubuntu Feisty Fawn. This could possibly be a problem here since it is a bit old. Maybe too old for this fast changing world.