There's nothing you can do. Flatpanels don't have real refresh rates. It all depends on the quality of the panel. What matters is the time it takes for the crystals to change colour. On cheap screens that time can be for example 40 ms (or worse) resulting in: 1 / (40 * 10^-3) = 25 fps
Good quality screens for example have refresh times of 25 ms (or even better) resulting in: 1 / (25 * 10^-3) = 40 fps.
So that second screen is likely of less quality than your first one. Playing with things like modelines is of no use and can make it even worse.