When I am driving my 96 jeep cherokee sport down the road I notice that when I speed up my odometer and speedo both go to zero then back up to normal and they do this about three different times going to 60 mph. But, when I stay at a constant speed or decelerate it doesn't do it.
It could be a bad vehicle speed sensor, lose wire, or bad ground. Check under the dash for wiring issues and grounds. Also, if the check engine light has come on, have the computer scanned to identify where the problem is coming from.