[300ohm]

Its the original CM4228:
http://www.digitalhome.ca/forum/album.php?albumid=586&pictureid=2975

I guess it is an unexplained local anomally for me

Yeah. It definately sounds like the leaves have something to do with it. Normally, when the leaves fall, reception gets better for most. My guess is you have a multipath issue that is blocked by the leaves. Also the GH is mounted where the house could be blocking the multipath signal or maybe the big heatalator pipe behind the GH is knocking away the multipath signal.