I added ent_purge for all entities that were created by ent_create before changing the level and now it works ... but I think I have to take a deeper look into that, because as fas as jcl told me I don't have to do that for entities created by ent_create ... but if I remove those lines the problem comes back.

@unknown_master ... it doesn't depend on the capacity of the video card, because the video memory gets less step by step with every level change. Even with 1GB or more it would get out of memory, sooner or later.