I have had a similar problem although not spectrum .I fitted a new orca speedo to my sons car and it had all the same problems as you have, fortunately when we tried my daughters radio a Sanwa mx3' and my radio Sanwa m11 there was no problem and then when I tried my sons again a Sanwa mx3 his still had all the problems that you have.
Try another rx / tx just to eliminate the speedo having problems ( and to clear your head).
Hope this helps!!
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