Nothing too scientific here, but I had similar problems awhile ago running Flightsims. I found that the problem was the Nvidia driver picking up remnants of an old installation if that makes sense. I fully removed the driver installed (through Device Manager, remove graphics adaptor) and then picked up a download that cleared any remnants. Restarted computer and then downloaded and installed latest driver from Nvidia. Frame rate went from 10-20fps to 60-80 instantly - and stayed there!
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