Just went through this a few months back with my '98 GC. One of two things. Heater core blocked, blend door not working properly. Mine turned out to be the heater core. I checked the two heater core hoses coming out of the fire wall. Both were hot. The core was not blocked all the way. But enough to make the heat turn cold when fan was going faster than on "low". Does the heat seam to work a little when you first turn the heat on (when everything is warmed up), then quickly turn to blowing cold when you run the fan on mid, or high? I did the flush/back flush thing, and it seamed to help at first (a little). But not for the long term. When you do the flush/back flush of the H core, flush the whole system too. Their is a lot of info on you tube about this. Some vids on how to fix.
Good luck.
Oh, by the way. I ended up having to change the heater core.