Monitor issue or graphics card ?

I have a Nvidia Geforce 8400 gs graphics card and recently had issue where nvlddmkm.sys was having a problem , which i fixed with the expand method , i still have random dots of colours mainly blue n yellow but alot of other colours poping up if im using applications or even browsing the web , what's the issue and what can i do to resolve the matter - Also some times screen freeze goes black and nothing