It's an issue with the speedometer bouncing. To stop the speedometer bouncing, unplug the antilock brake unit. It's located behind the glove box. You aren't losing anything by unplugging the antilock brakes, They really don't work that well. You can spend a lot of time chasing that speedometer bounce. This fixes it pretty easy.
1995 F150 XL, 2wd, I6
1995 F150 XLT, 2wd, 5.0