locky
Nobody has mentioned your original comment that it only happens on second sectors. Sounds like a clue. If this IS invariably the case, then the trick will be to decide what component might be working under different circumstances from the first sector - temperature most likely, but not necessarily. Are you always operating from the same few airfields? Could it be related to a specially long or short taxy distance or is it not airfield related. Even negative answers help to eliminate possibilities.
Anyhow, you stick at it. Aeroplanes are only machines and there will always be a logical answer to a persistent characteristic. Aeroplanes fail only because somebody has not done something right. In this case you are not the weakest link.