What can cause my (OD off) light on the dash to flash on & off on a 2001 ford winstar?

I took it to a mechanic shop to have transmission scanned. I forgot the code it showed but they said it was pointing to speed sensors. But the other guy at the shop drove it down the road and the scanner that was still hooked up shows the correct speed so that means that speed sensors r good he says, so they want me to leave one day and they will look for broke or loose wire. But I know that's going to cost way to much $ so if anyone has a solution please share?
BTW: the speed ohmedar sometimes only goes up to 20-25 mph. When I'm really 50-60mph. Until the OD light started flashing now it stays on 0mph. And sometimes it starts working correctly. The dealership replaced the whole speed ohmedar unit on the dash awhile back trying to fix speed ohmedar but that didn't fix it. And the cruise control stopped working also

Quite possible the speed sensor. Find local shop at ATRA.COM for more help