automatic? stick? you don't say which.

Overdrive relays are known to get intermittant. Did you try pushing the button to cycle the upshift light on, then off?

The solenoid on the transmission needs to have 12volt power to provide overdrive. A bad connection at the bullet connector will cause your symptom as will a failing solenoid. Although a failed solenoid will rarely return to operation, but can (I know this personally).