Weird graphics behaviour after installation. Debian

Installed on a machine with nvidia fx 5400. Using nouveau drivers. On log horizontally on the lower 3Rd of screen the graphics are a mix of lines and colours

This disappears after log in but if I view a txt window (terminal or half file) the lower 3Rd is blank, unless I drag cursor over it. Then it magically appears.if I scroll the window it dissappears....

Any suggestions as I don't know what to do given I am using suggested driver.

Cheers andy

Thanks andy.
Tonight I tried doing some updates and ran into more problems. So at that point I tried with a different pc using an ati card. All is good, installation and updated fine, graphics good. Even started exploring linuxcnc and getting mess card I/o functioning.

Cheers for your post.....

Andy