900 EPC controller with erratic PV values

I have a 900 EPC controller operating a climatic chamber, with process variable 1 (PV1) controlling the chamber's temperature and PV2 controlling the humidity (when required).
Approximately 2 years ago the humidity display started to become erratic, jumping ±8% around it's true value, e.g. Where the true value was 40% RH, the reading would This would happen for the first few minutes after the chamber was power on, then the the value would become stable, presumably after the controller had warmed up from cold. After a few months this value remained permanently erratic and the chamber could no longer control the humidity.
From then on I was only able to use the chamber for temperature tests, but the same pattern has now happened to PV1 so now the controller and therefore the chamber is no longer useable.
Is this a known fault with 900 EPC or similar controllers? Is there an easy fix for this or does the controller need to be sent back to Eurotherm for repair? (If repair is even possible).
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