Jimmy heater blower pressure problems

Here's one that none of my friends or I could figure out. My 97 Jimmy has just started to have problems with the blower for the heating system.
On AC and normal cool setting the blower works fine. When I turn the temperature gauge to hot, both the volume and pressure of the air being blown drops. This is with the vent or floor settings. Any ideas how to remedy this or what has caused this?
The vehicle recently had the cooling system flushed, a new head gasket replaced, and a new thermostat installed. Could any of this work resulted in this recent problem? Thanks!

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