Those "Short Bursts" of "Oven Like" air are your Mono Valve allowing HOT Coolant into the Heater Core.

Either Mono Valve Problems
Or CCU (Climate Control Unit).

Wild Chance.

Errrgh! Look at the 10 Amp Fuse(s) in the top of your OVP Relay
[Aluminum with a RED Plastic top] in the compartment behind the Battery.

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'84 300SD sold
124.128