Whirly,
I came across that about three years ago, when I asked a similar question to one engineer his reply was slow operation of the clutch, I didn't fly that crate again for some weeks and when I did it was much better, It was further explained to me the size of the motor used to engage the clutch was very small and could become tired with many start up's, but then another experienced user said dampness in the system could also cause it, IE heavy dew from early spring or summer mornings. another explanation was oil viscosity on first start up after very cold night if Heli left outside!
Keep looking though, cos its your bottom that will be exposed!
Vfr
PeterR-B