Monitor fault only on gt640. Working ok on 7900gs. What is the problem

Paulusheart

Honorable
Oct 1, 2012
66
0
10,630
As per title. Stripes on the screen when using gt640. When change for 7900gs everything working ok. Gt640 tested on another monitor and 100% working. What may be the reason. Checked another vga cable all the same. Another problem is picture becoming brighter and brighter from about 2 months time.
Please help.