--Thanks--First off, a big thanks for all the replies and suggestions. Further thanks to Iceberg for the testing - it's appreciated.
RE: the LNB being temperature sensitive, although possible, it is a new, practically unused (only for testing) unit, so I'm thinking (hoping) it isn't that.
--Update--
So this morning, I went to change the feedhorn, and noticed that the exisiting feedhorn wasn't straight in the scalar ring. I couldn't undo the locking grub screw (wouldn't budge), so I realigned it with my trusty multi-purpose adjustment tool
aka: hammer
I now have a steady 42%-45%, and no more dramatic jumps, so I am a happy camper
I'll see what happens tonight.