Some of my doors are acting wonky

Some of my doors act fine - made according to HammerWiki tutorial - but a few act bizarre. They slide up only part-way and seem to vibrate in mid-air erratically. Is there some issue with having non-BSP or other stuff above the doors that makes them not work properly?

Also, why do they provide a door prop when they just give you the door texture anyway (metal/metaldoor001)? Has anyone here just made a func_door using that texture and just skipped using the door prop? The problem I have with that door prop is it makes it such a fiddly mess to set up a door - and it's almost impossible to get the func_areaportal to sit correctly inside. (The prop is 1.7 units thick. How do you get an areaportal inside that? Last time I shifted it a half unit then clipped the other half off - to get the areaportal half a unit thick - but at least I know for certain it's inside the door prop.)

I think that guys means you can use an ordinary "begins open" areaportal.

Yeah, I've been putting in half-unit-thick areaportals. Make it a unit thick, move it up half a unit and clip it along the grid.

Plus, I found out why my door was wonky. I had parented the trigger to the door, not realizing that that meant the trigger would move with the door. (At that time I thought parenting didn't necessarily refer to physical movement, just "control".)