There may be a stuck vent door or an issue with the heater blower control head.
A duct has come apart under the home?
If your problem is that the cold air stops blowing out the AC vents and transfer to the defrost or the heater vents, I have had this on my 99 Astro. There is a small vacuum line that runs from the Drive Side Valve Cover near the front across the front of the engine over the Air Conditioning components. If this line gets cracked or broken, when you are going up hill and the engine requires more vacuum for fuel there isn't enough vacuum to keep the Air blowing out the AC vents. Check the line for cracks or broken areas.
It would depend on which seal. There are several seals. Some are easy to replace while others are difficult.
try rechargeing your a/c it could have to much or too little (added to answer) If the A/C blows cold at idle or while you are coasting, and stops blowing cold while you are accelerating or cruising under a slight load, it is more likely that you have a vacuum problem with the heater control panel, gates, or dash pots. If you ever hear a hissing sound coming from the dash, this is definitely the problem.
Yukon
This could be due to a problem with the ductwork, such as a blockage or leak near the air handler that is allowing warm air to mix with the cold air. It could also be an issue with the damper system not properly regulating airflow to the rest of the vents. Have an HVAC technician inspect and potentially repair the ductwork and damper system to resolve the issue.
Blowing nose while eating
To effectively vent your attic and improve air circulation while preventing moisture buildup, you can install a combination of intake and exhaust vents. Intake vents, such as soffit vents, allow fresh air to enter the attic, while exhaust vents, like ridge vents or gable vents, allow hot, moist air to escape. This creates a continuous flow of air that helps regulate temperature and humidity levels in the attic, reducing the risk of moisture buildup.
Yearly maintenance should include blowing out dust and then vacuuming up the dust. While the transformer is down all the bolts and terminations should be checked for tightness. Make sure that the vents in the case are not blocked by foreign objects that happen to be in the same room as the transformer.
Licking involves using the tongue to contact a surface, while blowing involves expelling air through the mouth. Licking is a form of tasting or exploring with the tongue, while blowing is typically used to produce sound or move objects.
The selector mechanism needs repair or the doors inside the vent are jammed.
blowing cool air, is the air only in your mouth... warm air comes from your lungs. temperature also varies by how open your mouth is while blowing air out