One other thing, are you using the gps to calibrate the u3s? The OCXO oscillates at a lower frequency, maybe the OCXO is set correctly and it's working it's way to its settled point.
I use a big calibration step and 240 seconds on first start up after the Argo setting up then once it's settled down to around 26.998 mhz or so it rarely needs calibration and after around 20 mins warm up time rarely shows a drift on wsprnet.
Hope you sort it soon,
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