RI will have wheezing, popping, clicking, labored breathing sounds with mucus in and around the nostrils (this will sometimes be wiped on the cage glass). You might also see yawning with stringy saliva. Sometimes a snake will also lay funny with its head pointed up so it can breath easier.

Rubbing its nose is not a sign. A snake may rub because of mucus but you should be able to see that.

Having a temp drop of as much as 10 degrees is not precise. This us why a tstat is better than a rheostat. The rheostat does not respond to temperate changes when a tstat does.

You should download a pic and find a night when you can try to watch the snake to see if you can find what it is doing.

I have heard of and seen snakes that rubbed but their is usually a reason. Being that this snake has been fine for so many years in that cage there has to be some unknown causing it. Your snake should not be very active. A happy snake is hiding. Sure they will usually come out alittle at night but it is not common for them to explore as if they are in a new home when they are not.