1998 Chevy Corvette need help

First thing to check is the exact refrigerant charge. You can't rely on the pressures and they have been known to act that way being as little as a couple ounces low on refrigerant.

If that checks out OK, then you probably have a problem with the blend door actuator or the blend door itself on that side of the system. If you can access the actuator, see if it is trying to respond when the temp setting is changing.