I have had similar issues once in my 2wd G' ....... When I found the problem I felt such a prat for not noticing earlier ...... the fan wires on the speedo ... one had broken at the solder joint and in some cases was breaking / making contact and arcing .... the voltage spike was tripping out the Rx.
Was a Nosram speedo too so worth a careful look as you can't see the solder connections under the sticker!!!
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