I don't know where these guys are buying their hair driers from, but the store manager probably appreciates their business. I've never seen a hair dryer yet that doesn't depend upon unimpeded air flow to keep the heating element at a safe temperature. Impede that flow and you'll either melt the housing, trip the internal self-resetting thermal cutout or blow the "emergency" thermal cutout that must be replaced at a cost higher than the dryer's worth.
An Unger heat gun, although expensive, is designed for this kind of abuse, although prolonged use with some of its more restrictive air stream attachments can melt the end of the gun as the tip heats up.
If a hair dryer is all you have for heating, then don't impede the air flow. Instead, heat up an area until the problem appears (or disappears) and then use a more selective freeze spray to cool small areas.
And good luck. Thermal problems are some of the most difficult intermittents to find.
Dean