I have had the same experience and it was resolved by a Raymarine tech and he didn't know how he did it.

I wasn't going to add to this post since I can only guess, but here is what I think happened.
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Bottom line was a loose connection caused by a new Radar installation.
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The Tech took out the ST60 display and bench tested it at his shop.
1) He said he opened it and found a "lose solder connection" which he fixed for free.

(translation: I couldn't find anything but wanted to blame it on old equipment not on his recent installation)
2) Told me it was the Transducer at top of mast.

(Translation: It can't be my installation)
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This story continues until finally they knew the Radar was not consistently working and did what they called "shot gun approach" and rewired everything that was new.
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My old ST60 wind now reads True and Apparent (the knot meter must be working on SeaTalk also).

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Last point which I learned with chatting with a very well trained Raytech. The wind speed at top of mast , if spinning, is so simple, it is hard to fail. It is just like your knot meter, an encased magnet on a wheel that passes a static sensor and counts the revolutions as pulsed DC. The conversion work is done in the display and then sent over the SeaTalk network.

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There are at least 4 connection points. The one at the top of mast was never touched. The 2nd was at a buss for wire at mast base. 3rd power breaker at panel. 4th the ST60 display shown in post#3 by GGordonWoody.
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Good luck...
Jim...