I have a 92 XLT.The temp guage has always read cold but the heat was great.About a week ago,I heard a rattling noise coming from the area of the glove box.The other day,the heater started blowing cold and the temp gauge rose to about the R.When the gauge suddenly dropped,the heat came back.It does this constantly now.Gauge goes up,no heat Gauge goes down,heat.Does this sound like the blend door or something else?
Mick

Honestly, I don't really know, but I was having a similar problem with mine. This guy said it could either be the core itself, or the water pump going out. That didn't really make sense to me, but that's what he told me.

Ok,I understand the core part(partially blocked?) but water pump?

The fact that the truck's temperature is fluxuating tells me that you could have a thermostat that is screwing with you. Also, if it's doing such drastic things like slamming shut, then open, like you are describing, you could have an air pocket. I'd figure out why the truck's temp is all over the place, then address the heater. Check out the Thermostat.

Thanks,I'll check that and I guess it wouldn't hurt to give it a flush while I'm at it

Not a bad idea. Who knows what you'll find in there. :)