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Eugene, you are not alone in having this difficulty early on in the evening.
I have 18" F4.5 so it takes quite a while for things to stabilise - its kept in the garage by day and right now as the sun is behind the trees, it is out trying to cool down with the evening. Of course the night air is always dropping faster than the scope. I have a full set of dew heaters which do help later on, but the biggest help I find with double stars is my off axis mask. I have made a 7" one which makes the scope work at around F11 and really improves double star performance. I suggest you make a 4" one that sits between the secondary vanes. Your scope will work at F12 - focus without the mask and then set the mask over the tube. Instant best focus spot!
My mask is a bit of wood veneer, painted black with Velcro fasteners.
Cost is nothing so give it a go. It does not work so well on very faint stars. Many commentators have scorned these masks over the years, but I find disagree with them and reckon they are not observing under difficult conditions.
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