Intermittent steering hardovers really sap command confidence, and can be devils to track down, so you have my sympathy.
Is there any pattern to when this happens?  Has it happened enough times to know whether any pattern is significant?  Does it happen when people are doing breakdown drills?  Is it always the same helmsman it happens to?  Is it always the same control system, or the same pump?
Is the performance of the gear adequate the rest of the time - good positional accuracy and sweep times?
Before interfering with anything else, I'd have a quick look at the return line filters in your steering hydraulic system and judge from the state of those whether there's much chance of the problem having been caused by valves having got stuck open by debris.
Assuming the filters are clean, I'd then start going over the ship's cabling - starting with the rudder feedback signal, then along the FU demand path from Hand Steering Unit to Servo Amp.  Tug twist and wiggle the cable at every loose loop, every plug break (especially!), and every bulhead penetration to see if you can recreate the symptoms.  Once you've turned your intermittent fault into a hard fault, diagnosis becomes much easier.

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