transmission is stuck won't shift out of park.

My wife's '99 new beetle (automatic) is having occasional trouble shifting out of park. It's like it doesn't register her foot on the brake. Sometimes it works if she takes her foot off the brake and then puts her foot back on the brake and tries again. Other times it takes several minutes. Sometimes she has to turn the car off, wait, and then it works.

I don't know if it's an issue with the brake sensors or the transmission, or what. Any ideas?

Thanks in advance.