5.4 engine in HD F-150 - When towing my travel trailer I have had a dozen or so episodes when speedometer becomes erratic. Once it stopped registering speed and stuck at 80 MPH (I wasn't towing that fast). I disconnected neg. bat. cable for a few minutes and it reset and operated normally for a couple of weeks.

Next time I pulled out of gas station after fill up and back to highway speeds and noticed everyone flying by me though I was reading 60 MPH. Radioed traveling buddy and he reported I was going 40 MPH. Same cure.

Sometimes it will still show speed increases sometimes it won't. Only does it when towing so since returning home it works correctly so I haven't taken it to dealer yet. Any ideas?