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When under strain, like going up a hill or accelerating, the AC will weaken and stop coming out of the main vents;
Sounds more like a vacuum problem, if the airflow fades to defrost under load then the diverter door which is vacuum powered is losing vacuum. Prime suspect parts are under the hood though, they include the vacuum resovoir(juice can on the passenger inner fender, rigid plastic vacuum lines particularly the white one that goes into the firewall near the blower motor, and any check valves(round black plastic disk) in the lines
It does sound like you have a vacuum problem. An EGR valve that is sticking open can act like a vacuum leak, causing vacuum leak-esque problems. You need to check the vacuum check valve in line on the vacuum line feeding the hvac system.
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