I've tried it on two video cards and both seem to do it
they are XFX 4890 1GB
GeForce 8600 Gt
Phenom II X3 720 BE
Corsair 750W PSU
Obviously a big difference but the problem is the same.
Basically I have draw distance problems with objects just popping up on screen. When this happens I take a big FPS hit. I recently bought a new MOBO + Processor. I'm fairly certain this problem existed before I bought them so i'm really not sure what the cause is. Another is shadow flickering in Fallout. If I move a certain way a random shadow will flicker and sometimes lighting on an object will go completely dark. The draw distance problem is in Fallout as well.
Sorry but im going to start with basics so dont be insulted, but. You do realise there are settings for draw distance in the game settings, right ? Oblivion for sure, not 100% sure with Fallout.
If you increase the draw distance and in oblivion there are fade settings also which will help.
Try reading through this http://www.tweakguides.com/Oblivion_1.html
hopefully its just a case of setting things up properly.
Check in your drivers that you have them set to application preferance as well.