instant economy gauge stays at 99
 

I have a 99 Grand Marquis LS with the digital cluster. The instant economy gauge will top out at 99 and stay there....fist it goes from 50 then 75 then 99 at a dead crawl. Then all of a sudden start working normal again. This happens daily and it's driving me nuts. Sometimes I can go for miles and it will work normal then start acting up again after I stop. It is also affecting the average economy gauge as it's showing 22 mpg average and I know it's not getting that.Any ideas?....Thanks