I would also investigate whether there's undesired airflow:
- The seal b/t the front air handler and the cowl (pictured) is notorious for leaks; it leaks on my Suburban, which is very evident in the wintertime b/c it allows a cold draft of air to enter behind the glovebox into the cabin. Certainly in the summer there's a warm, humid draft.
- The seals b/t the dampers within the air handler may not be routing the air completely though the evaporator / around the heater core
Others will hopefully comment.
@east302 has posted often about air handler issues / rebuilds, as have others.
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(this picture is courtesy of someone else; I pirated it years ago, possibly from someone on GMT400)