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Looks nice...
At the end of the day, are you any closer to tracking down your flexture issue? Are you certain that its flexture? or could it possibly be a balance issue? I find with bigger scopes (newts and long refractors especially) that whilst your RA and DEC might both be fairly well balanced when the RA axis is horizontal, when the RA axis is closing in on the vertical position, the balance changes quite a bit... This sounds like it could be your problem, I figure, if you can get 4min subs at the start of the night without problems, but as the target gets lower your results start to falter, either balance or perhaps poor seeing at lower altitude is the culprit.
What exposure duration were you using for guiding?? as the target sinks lower towards the horizon, I tend to bump the guide exposures up to 2~2.5 seconds.. with shorter exposures, your guiding can chase the seeing, giving the appearance of poor tracking and or flexture...
Just other things to consider I guess...
Best of luck chasing it down..
Oh, are you using adjustable guide scope rings or solid rings? If adjustables are being used, remove them, smash them up a bit with a hammer, then throw them in the bin, get some solid rings for your guider.. Those adjustable rings were designed by the devil to make people cry.. Pure Evil.
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