Hi Phil,
You indicate an electric choke, which if the usual variety is simply a 12V source to a heating element within the choke housing that heats the spring which unloads the choke butterfly... it may take a little longer to open fully in colder temps but the overall choke action is irreversible as long as the choke sees a 12V source i.e. as long as the key is in "run" position. So yours should not be able to remain partially closed after driving awhile... my 2c worth is that your choke is malfunctioning, losing its voltage source, linkage sticking, heater element failing, one of those type things
With your fast idle staying up I assume the choke fast idle cam is still active confirming your choke is not releasing fully. The choke release is independent of your thermstat setting. Check your choke butterfly and linkage - wiggle it open/close with the throttle unloaded to check for binding etc - put 12V to the choke to test its function. Make sure your 12V source remains contant to the choke, no wire probs etc. The electric choke is simply an "on or off" system, so when "on" has to open fully within a set interval of time if it is functioning properly. Good luck!