Sinus problems at altitude

The last couple of flights where I have gone above 7000 ft, I have
experienced this strange physiological effect:

My left front teeth, gum, and upper lip go completely numb, exactly as if I
had gotten a shot of novocaine. It stays that way until I start descending.
This happened to me once on an airline flight years ago, and was accompanied
by a splitting sinus headache. The two recent events have been pain free,
but it's disconcerting to have parts of your body lose feeling while flying.
I had a linering low-grade cold during the first flight, but it had cleared
up by the second. My father (a physician) had never heard of anything like
this, but hypothesized that my sinus is swelling up and pressing on the
facial nerve in that area.

So, has anyone experienced anythning like this, and what can I possibly do
to stop it?

Jeremy