Nose gear doors opening in flight

[MelsML55]

A short time after take-off the nose gear doors fall open. Pull them back up, 2-5 mins later....or sooner.....gear doors open. Actuating cylinder has been rebuilt 2 times. Curiously, with your fingernail between the gear doors you can pull them down without any effort. Master switch and landing gear handle in ANY configuration.....same result. You can pull the doors about half way closed, then the resistance will not allow closure. Slight hydraulic film on the belly, likely from over filling the res after refitting the actuating cylinder, no leaks detected under the dash. 1965 337 number 0199.
Fire away.