Electronic Readout on Ceiling Interior

1997 TP, 3.4L, 170k miles, actual miles per gallon is between 18-19 mpg, the readout coming from the LED unit (mounted on roof interior about the rear view mirror) goes haywire when it's on cruise control on smooth ground at sea level - starts going from 16 to 30, then back to 16, back to 30, hangs for a moment then starts the cycle all over again.