Hertsovka monitor and image quality?

Question

Iron

Hello! Although I have an old monitor, but still LCD.
There is a standard resolution of 1280x1024 (60 Hz), but if I set it to 75 Hz, then the graphics become fuzzy and muddy a little. In a word, it is not very comfortable both on the desktop and in games. But on the other hand, the fps is increased by +15, if I set it to 85 Hz, then + 40fps, but still fuzzy. Why is this possible? Or is it just an old monitor? Maybe some setting? Has anyone encountered a similar one?
Of course, I understand that the monitor has already outlived its own, but still.