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Unfortunately, that was what I was doing: but thanks for your input. I feared no one would have the expertise to advise.
I'm not experiencing it jumping and juddering as such. It's that it's become noisy, and instead of cutting away the wood smoothly as it first did, it turns out it's wrenching off inch long (or larger) slivers, somehow.
I did remember to check the 3 fixing screws this morning, but that was a dead end. They are in as tight as ever and the router is fixed & stable in the table.