EGA was "dual-sync": in the 200 line modes, it ran at 15 kHz, whereas the 350 line modes ran at 23 kHz. While it's quite possible that the card had the problem

EGA was "dual-sync": in the 200 line modes, it ran at 15 kHz, whereas the 350 line modes ran at 23 kHz. While it's quite possible that the card had the problem, it's also possible that CRT was the problem. If the components drifted out of spec, it might no longer be able to correctly sync at 15 kHz, especially after warming up.