instant economy gauge

I have a 99 Grand marquis ls with the digital cluster. The instant economy will go to 99 and stay there then all of a sudden start working again....this happens daily. Sometimes I can go the whole drive with it working normal then stop at a light and when I go to take off back to 99 and it stays there....this is affecting the average economy gauge also....what could be the problem?...speed sensor ?? ideas please it's driving me nuts....Thanks