Gingernut,
Yes, seems to be. Sometimes comes on with a cold but not often. More usually induced when flying. Recently had a bad attack of sinus pain whilst flying, no problems on climb-out but after half an hour in the cruise, unpressurized, at FL100, severe pain down one side of my face. This eased on landing and the application of some painkillers.
I think on this occaision I must have had the lingering trace of a cold which provided something to clog the sinuses. Have found that using a dehumidifier at home really helps but obviously no good when away for a week sleeping in overheated hotels.
Might try the bicarb of soda. Not sure what Customs might say though if I was found carrying a bag of white powder.
Any ideas?