Problems with 60hz resolutions

[zerozerosette]

Hello,
I've recently bought an arcade cabinet. I am unsure about my monitor setup (it is a 15khz/25khz crt, don't know the exact model but if it is required I can post a picture): I have an old radeon 9200 with soft15khz (but I tried crt emudriver and nothing changes) and groovy mame. If I choose a resolution of 640x288@50 the screen is completely flicker free, while every resolution with a number of horizontal line lower than 288 (240 for example, which is 60hz) produces a less stable image. The effect is less noticeable than an interlaced resolution flicker, but I find it... annoying. Is it normal? Is there something I can do?

Thank you!