Probably a dicky indicator bulb or connection. Try identifying if one of the bulbs doesn't flash for the first few seconds. I had a similar problem, you could tap the indicator unit and it would start working. Intermittent faults are always hardest to find. I changed the bulb and it's been ok since. HTH
I had the exact same thing-drove me chuffing crazy!! Try giving all 3 connections at each lamp a clean up and spray with WD-40 or similar. With mine it was the earth on the rear lamp connector plug, it was a little corroded so i cleaned it up with a bit of emery cloth sprayed it and its been fine since. hope that helps!