Satellite signal levels - different problem

I have a 80cm dish to receive Hotbird. The signal on some
transmissions - for example NPR radio - varies from good (quality 70
percent, level 60 percent says my Fortec Star receiver) to unusable.
The Astra 1 signal, who's LNB is off axis, is consistently better.
Both are using the same type/brand of LNB.
I've turned off my DECT phone (thanks for that tip).
It seems not to be the weather either.
I live in the south of England.

Any suggestions?

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