As simple as it sounds look for debris around the fan intake. I had a similar problem with low flow and had a plastic bag around it. Also does your fan sound like the bearing is bad? if none of those you can take off the heater case to the left of the fan which stores your heater core, if you look up past your core with a flash light you should see a door that actuates with the heater switch I believe that door is vaccum actuated. I'm not sure if this will cure it but probing may allow you to see somethign you haden't seen before