Topband: Effect of trees

[Tom W8JI]

<<<The difference between winter and spring/ autumn and winter usually is
very distinct up here and moisture is quite constant throughout the
year. I have noticed that I have to  adjust the phasing when temp. is
going above zero and (in fall) it´s been below zero.
Sometimes spring is late, sometimes earlier but the difference in
phasing always is there.>>>

Perhaps you have a temperature sensitive component in the phasing system? 
That would explain the change.