Probably the texture compression format you are using? its 4.25 its A LOT more stable, in 4.24 i remember the terrain collision wouldnt update and all that jazz, and on level load, or placing an actor on the terrain for the first time can cause it to get blocky (geo blocky, not texture bit blocky) for a few seconds while i assume the landscape is initialising itself again, but again only ever once per after loading the level.