Based on 27MHz I¡¯m assuming you are using a crystal reference synthesiser rather than the TCXO
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its possible the crystal or 5351 is failing but try setting the frequency back to nominal and see if it transmits on frequency OK and cleanly ?
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Also try another GPS antenna if you have one and see if the GPS attribute ?display looks good?
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Beyond that the cheapest fix that also an upgrade is a new synthesiser board with TCXO option as an easy-build ?plug in swap ?(just needs the freq changed to 25MHz in the Ref freq menu?
Bruce Akhurst