Update for those interested. Finally gave up and took the truck to Ford for repair of the problem. They drew no conclusion either as to the specific cause, buuuut...., the technician noticed that a portion of the wiring harness connector was melted and the wires coming off the battery side of the harness were bare and damaged. So, he wound up replacing the blower motor and fan assembly, resistor, and wiring harness. So far, the problem has not returned. They saved me the parts and, upon closer inspection, I was able to observe where the 2 blower motor wires in the melted harness connector were shorting together. The harness connects to the blower motor and resistor, on one side, and to the underdash wiring harness on the other side. This is where the melting had occured and I just didn't tear the dash up enough to have gone that far in to find the problem at the other end of the wiring harness. That still left the wonder as to what happened to the harness that caused the melting. I could only conclude that something may have happened way back when I replaced the blower motor and maybe this had been there since then, just popping fuses whenever the wires touched from vibration. So, in the future, I think I would first assume that a fuse that pops like that with a lot of heat in the wires is more than likely a short somewhere vs anything else. Thanks again for all the opinions of repair. I think the last from Bluegrass7 about a possible fire probably urged me to know my limits at that point and have someone else take a look.