Intermittently downshifts for no apparent reason

1996 ford fairmont ghia 4.0 ltr 6 cyl. Intermittently downshifts for no apparent reason while driving. Sometimes selects neutral gear while cruising. These fault appears to be more common while the aircon is running \

My car has a tendency to shift gear erratically while driving at 100 kph on the freeway. It doesn't appear to have any other fault in the transmission and drives quite normally most of the time. It does, however, seem to be very sensitive to the slightest acceleration and will downshift with little reason. The problem also appears to be more frequent when the aircon is running.