Another thing you could try out (don't know if it will work everytime, but it has for me for some reason) is move the .fhm (or whatever. Basically the one that is the portal itself) to textures. Like I said, this is a long shot and may not work, but I just wanted to throw it out.
And I know Buffy has a portal mod, just go to the mods board and you'll be sure to find it. There are also a couple on DA.