This may be a) the A/C head acting up, b) the heater blower resistor acting up, or c) the blower motor itself acting up.
Good news is, b) and c) are often enough, it may not be a bad idea to swap both out.
Now, are you a bit of a contortionist? What I'm about to say is not really for fat white boys (raises own hand!) or people with bad backs, but I did manage to do it with the seats in.
On the passenger side, under the dash, is a sound deadening panel. If you remove that, you can see the blower motor and the resistor pack to give you various speeds.
At that point, it's easy to make sure it's not something really silly like a bad connector - or you can turn the heat on and thump the blower motor and see if it comes back on.
Four bolts, the motor comes out - buy a new or reman'ed one with the squirrel cage on it, and it's four bolts back in, reconnect, and good to go.
The resistor is right beside the blower motor, connector and two bolts it's out, two bolts and connector and the new one's in.
Both should be available for under $100 easily.
RockAuto has several vendors for the motor with the wheel ... http://www.rockauto.com/catalog/rafr...&parttype=6916
They also have several vendors for the resistor pack ... http://www.rockauto.com/catalog/rafr...&parttype=6557
Given shipping, you should hit right at $100 for ACDelco for both. I'd not buy too cheap - that position can hurt your back while you're in there (I do it through the open door, skinny folks that can touch their nose with their toes can do it in the front seat *grins* ) and it's not something you want to do too often. After all, the AC Delco has lasted, what, 14 years?