Warm air mixing with A/C only when driving over 50mph?
You could have a vacuum leak or a worn out engine. Low vacuum readings can cause a blend door to “relax”. You can “crutch” this for the warm weather by putting a shutoff valve in the heater hose under the hood.
I much preffered the old Chrysler method of controlling temp with a heater control valve and the heater /defroster operations with cables.