Then you obviously haven't flown fifi enough. Not only does the POS vary plus or minus 20 kts it will sometimes just give up the ghost and not even try to make an altitude. Hell, it even will decend below hard altitudes going into Denver. Known problem and within last 2 months there was a bulletin about it. Absolutely horrible vnav. Not to mention it doesn't decend out of TOD on its own which is a complete joke.