a/c stops blowing hard out of registers,why?

After my ranger gets warmed up for a while, my a/c stops blowing hard but
you can still hear the blower motor at the same speed. It does not matter if
you try the defrost, floor or front registers, you get just a little air coming out.
The blower motor, resistor and relay have been replaced. If you let the
vehicle sit for a while and cool off, then everything is back to normal. It
seems to me that it has some kind of blockage that occurs after it heats up,
is that possible and if so what could it be?