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>Oh bugger. I had naively taken it for granted that the quoted' wide
>operating range of 2.0 to 5.5V' was for all operating speeds... and looking
>at the datasheet it seems this isn't the case..
>
>According to the datasheet, I'll need at least 4.5V. I think you may have
>hit the nail on the head!

It may not be the answer you wanted, but it sounds like an answer.

CMOS logic gets slower with lower supply voltages, so you always have to
watch for that.
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