[85Bowtie]

It sounds to me like you have air in the system. Are you sure you're getting all the air out? What you will have to do IIRC (been a long time since I looked under the hood of mine) is jack the front of the truck up so the radiator is above the intake. Open your radiator cap and run the truck up to operating temp with the heater on high. Any air in the system will go to the highest point, which is at your radiator cap.

Is the overflow filling up at all? If so, you may have a bad head gasket. If it is a small leak on the cylinder side, the pressure from inside the cylinder is enough to keep the coolant from running into the cylinder. Instead, it will push air/exhaust into your coolant system because of the greater pressure. This will displace t he coolant in your system and give you an air pocket which does exactly what you're describing. If so, when you have the heater on you would notice it blowing cold all the sudden.