I would first re-seat RAM. If you have to modules / memory sticks, remove one and try with another. If you have one, you can borrow another one from a friend or ask in a nearest local computer store.
Alternatively, you can use
memtest, burn it in a USB drive, make it bootable and boot your laptop using the created usb bootable drive. Let it run till it display "100% Pass complete" at the bottom of the screen. If any errors come up, they will be shown on the screen in red color.
If no errors are displayed, then I would move further to checking of the display cable (which connects screen and motherboard). This problem can also be of the LCD/LED screen going bad.
If all the above tests are good, then as computer_commando said, can be a graphics adapter/chip problem.