Your posts have me confused. Your first posts suggests you get no heat below the 80 setting,(only cold air blowing) but heat above. Your next post suggests you get no air blowing.
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Doesn't sound like a heater core,You'd get no heat
More likely a temp sensor or computor issue to go from cold to hot like that
I have an 98 with the auto/clim. Same symptoms, below 80, cold air. Above 80, will cook you out of the car. After searching this forum, suspected heater core "partially" clogged. Know issue, with rad caps, dexcool. Drained and flushed the sytem, and backflushed the heat core several times. Replace the cap and dexcool with green. Works like advertised. GM acutally has a TSB on this, they want $400 for a completed system acid flush with no guarantee it will fix the problem.