What causes monitor flicker?

Question

Iron

Good day to all, the situation is as follows:
I did not use the monitor for about a year, I did not change anything in the PC. Monitor, by the way, with VGA connection. Now it has become necessary to use it again. In normal use (web surfing, programs, etc.) there are no problems, but in games, by the way, not all, there is some flickering. More precisely, it feels like horizontal stripes are running. This is observed in new games, in old ones - unnoticed. Enabling vertical sync saves the situation, but this option does not suit.
What actions were taken:
1) the drivers were demolished, rolled back and stuff like that
2) the hertz was checked in all possible places
What could be the reason for this and how can it be fixed?

Answer 1

Either this is a banal subsidence of FPS and the manifestation of brakes in the game, or just the monitor matrix itself is slow.

Answer 2

The problem lay in the 760 Ti video card, replacing it - the flickering effect disappeared.
I still can’t understand what was the reason, given that the PC has not been used for almost a year.
computer config:
i7 3770k
nvidia GTX 760 ti SLI nvidia GTX 650 Ti BOOST
8 gb
512gb SSD
Monitor:
Samsung syncMaster B2030

Answer 3

This is due to the fact that the video card has finished generating a frame at the moment when the monitor was updating the image. It noticeably happens if the fps is low and the difference between frames is large enough to see such a gap, or if the frame was processed for a long time and was displayed at the time of the update.
It is solved by reducing the graphics settings in games to increase fps, turning on vertical synchronization so that such overlays do not occur, or replacing the video card.