Did you ever discover anything else about your problem? I have a 97 F-150 with the exact same problem. I changed the heater core and coolant myself last fall, and the truck's heater worked perfectly until this past Friday night when I lost the vast majority of my heat in the middle of a 3-hour drive. I immediately assumed the thermostat was my problem. The temperature gauge remains in the same exact spot it has for years--just below halfway but slightly above 1/3 of the way (follow me?). I purchased a 195 thermostat at Advance and swapped it out this weekend. Now my heat is better, but obviously not like it was before. I just stopped driving my truck about 20 miles. I immediately raised the hood and checked. The heater hose going into the core is hot, and the output hose is hot, but slightly cooler than the input hose. But I was able to grab and hold either one without much difficulty. The same exact scenario of approximately 5 seconds of good hot air before it cools down happens with my heater too. Any more ideas? Could my problem have originally been the thermostat, and now I've created an air pocket because I had to drain some of the coolant to change out the thermostat?