I had the same symptoms as you describe and was half way through my flight training at the time.
It always happened at the later stages of descent(in pressurised aircraft) and even happened once on the way back down a mountain in a cable car.
After quite a lengthy investigation process it turned out that my sinus had narrowed over time. A day procedure at the local hospital to widen it has seemed to have done the trick.
Try all other remedies first but sounds like a sinus issue.