Here is your wiring diagram
http://www.servicematters.com/docs/wiring/Wiring%20Sheet%20-%203399125.pdf

Which fuse is blowing?
There are two in this unit.
One on the blower assembly and one on the heater.

Since your problem is intermittent, it will probably be a pain to find.

Sounds like you have eliminated the vent and blower systems as a possible cause.

The first part I would replace is the operating thermostat, perhaps it is sticking closed every so often.

Another possibility is that the heating coil is sagging and touching the case every once in a while and then ope again once it cools down. The unit may then run at high heat and the thermostats do not control it. Check the inside of the heater case for signs of a short re: an arc mark. The problem with this possibility is that usually it blows the thermal fuse on the heater not the one on the blower.

Check your lint filter. Sometimes they can get coated with fabric softener residue and greatly reduce air flow. Again this is a long shot as you have checked the air flow outside and it is good.

These fuses are meant to blow due to temperature but will also blow due to high current. There is always a possibility that a motor winding is shorting every once in a while but this would also be a long shot.