got some weird problems with COD2
 

Ok.

I was just playing COD2 with almost hight settings and everything was just fine no lagging and such. I cleared the level, and waited for the other one being loaded, then the trouble begun. everything was going like real slow, cannot do anything, so I changed the settings. It was a little better but not enjoyable.

I can't figure out what the problem is.
I got a AMD 3000+ Athlon XP processor
1 GB memory DDR 400
MSI Starforce Geforce 6800 128 MB

I updated my video drivers, but that din't work

Isn't the game just buggy like that?

well with the multiplayer I don't have any problems, only with single player

Running it in DirectX7 mode works pretty well.

As I said - the game is just buggy.

Your computer may be overheating if there's a random slowdown. I have the game too, and I haven't experienced any slowdown on any specific levels at all. Does it only happen on one level, or does it slow down at a certain time after you begin? I've got:

Pentium 4 2.8GHz
896MB PC3200
GeForce 6600GT 128MB

Runs fine on my computer throughout the entire game.

EDIT: Are you using ForceWare drivers? If so, you might want to consider switching to other drivers. I used to have some horrible speed with CS: Source, then a friend directed me to Xtreme-G drivers for nVidia Cards, boosted my Framerates up by like 10 - 20 FPS. Check it out...
I believe the site was http://www.tweaksrus.com