When I turn the heat to defrost, the air sometimes seems very weak out of the defrost vents. It them seems to kick into gear randomly and blow normally. I can switch to the normal vents (like I'd use for AC) and it blows fine out of those.

The fan sounds fine. Brand new serpentine belt and filter.

Resistor? Blend doors?

 

I would guess the actuator and or the flap that directs the air between floor/vent/defrost is your problem. If it was a fan or resistor problem is would happen in all positions