Tranny light flashing

After driving about 4 to 6 miles the tranny light will blink .This is when in the overdrive position on shifter.When in standard drive position on indicator the light never comes on .This is telling me the overdrive may have problem.When driving in the od position it appears the overdrive isnt working you cannot feel or see the shift into overdriveon the tach.
My question: Is there a speed sensor or other item I should be looking for to correct problem. Had it to repair shop and they claim no codes show up but then light never came on because they didnt drive it far enuff.If you shut off engine it will go out but drive another 4 to 6 miles and it comes back on .