Overdrive light comes on flashing periodically on my 1999 F150.?

I have a 1999 F150 Super Cab and for a while the overdrive light has been flashing. I have hooked up my computer reader and it comes back as normal, no numbers for problems. I did some reading and thought maybe the sensor had sand on it, because I live on a dirt road and the puddles of course splash water. I got underneath and rinsed entire area one time and the light stayed off for about 3 weeks. Tranny fluid is fine. The truck only has 65000 miles on it and is in immaculate condition. It does not shift hard or lag at all. Anyone have any ideas. Thanks to all for assist.