8400 Problem!

Hi, I have two problems with my Nvidia GeForce 8400 GS, brought 2 months ago.

1: When I start the computer, it starts but I can’t see anything on the screen, if I restart it, then it shows up.
2: When working, the screen goes black and then it comes back. Like, I am typing, and the screen goes black and then it comes back again.

What shall I do?
Quick help is appreciated.
Thanks, Ratul

Update drivers, check Bio’s to see if onboard is not checked..

What are the specs of this system?

I did everything you said, still its happening!

Specs:

Intel Pentium e2180
2GB DDr2 RAM
Nvidia GeForce 8400
Asus P5GC-MX/1333

Power supply? Brand and model?

What were you using before you got the 8400GS?

The 8400 GS got finally destroyed and my computer mechanic instead gave me a GeForce 210.