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I don't know what the c9.25 mirror/cell interface looks like, but on the C14 a large retaining screw is responsible for holding the mirror to its cell (together with RTV [silicon adhesive stuff]). I'm afraid the only way to check is disassembly, so I would leave this as a last resort - partcularly as it is the most unlikely of the three.
Check the secondary holder's rigidity to the corrector by physically trying to wobble it from behind. It should be obvious if it is loose. Also, check that the secondary mirror is fixed securely to the holder - by whatever means the C9.25 uses (probably a combination of the collimation screws and something else). You might need to take the corrector off for this - so be careful, and seek some advice on this if you are unsure.
Yes - mirror flop is an inherent design issue with SCTs. There's not much which can be done to address it other than reapplication of the damping grease used on the central tube, or a mirror lock down solution (or both).
Other C9.25 owners may have some ideas worth looking into, but in terms of generic collimation I'd check for the obvious. For example, what about your eyepiece / diagonal ? Is anything moving around back there ?
Paul
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