Please help.

I have an inspiron 6000 (ATI x300 128 Meg)

I have an external dispay of a Dell 2405 FPW

and I just received my Windows Vista Home Prem.

 

I get everything installed and it works ok. However when I view with my external display, There is a screen jitter on the screen. It starts from the top of the screen, jitters left to right a few times very fast, then goes down an inch and does it again, all the way down, then starts over.

 

I have updated all drivers...What could be causing this?

 

Thanks