Unfortunately, it doesn't work as expected but there is no glitch when I played with the potentiometer.

For example, at 100.0Hz I obtain an 9.5Hz output,
at 110.0Hz -> 17.6 Hz
at 90.0Hz -> 6.5Hz...

And there is no way to use this? http://www.picbasic.co.uk/forum/showthread.php?t=12433

or this?
http://melabs.com/resources/fp.htm

(just a suggestion)