Which suggests that the anemometer transmitter is either putting out a weaker signal or there’s some new interference in the area. The weaker signal option seems less likely as you’ve used a different anemometer and it’s unlikely that both would have exactly the same problem. Tracking down interference isn’t easy though.
When I get the new solar panel I’ll try that with the old anemo first, then the new one. . .
After that, I’ll start asking 'round the neighbours 
It could be something in your house. It’s an unpopular way to test, but turn your own electricity off to shut most mains powered things down and see if the problem goes away.
That would mean, at current disruption rates, turning the mains off for about 12 hours (MTBF).
Recent additions here - on the same side of the house - are PurpleAir and Gaia sensors, but I’ve turned them off for long periods with no apparent amelioration.
you may need to play around with the angle of the aerial on the console
At current electricity prices you’ll probably save about £2000 by doing that!
And that’s just the standing charge 
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Had a week where everything looked fine, but now it’s cold I think I’m getting occasional flatlining. . . can be difficult to tell with such light winds.