Try whisking harder and pouring slower to achieve an hollandaise that isn't split.

If you whisk by hand then twist a kitchen towel into a diagonal tube and make a ring on the bench with it to hold the bowl so you can whisk harder.

Also, don't overheat it when keeping it at temp. Normally just leave it sitting over a pot of warm water or on top of an oven with a barrier to keep the direct heat off it.