I am trying to create a cobblestone room around my portal in the nether. But every so often it decides that portal blows and next time when I port into the nether, it creates a new one nearby the old one. Its really odd, at random times the purple way gate thing just goes out and then I know when I die (lots of ghast all over so I die fast) all my work was a waste since it will just create a new portal.
Hmmm. This is most interesting.
Shortly after the Halloween update went live, many people had this bug. Along with another that randomly spawns portals in the "normal" work. A patch was released that supposedly fixed it. Try re-downloading the ".exe" for Minecraft from the main page. See if that fixes it.