had the same problem on my 851 , as some other member will know.
random cutting out at any speed etc, flip the kill switch off, then on again quickly, back to normal.Can be a bit hairy on a motorway in the fast lane when it happens
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I tried every thing without success, when it went into baines, it turned out to be a dry solder joint in the managment module under the seat. problem solved, but it took a while to find it.