This is a new to me used car. The a/c has been charged, condenser works, belts fine. The a/c blows cold, the after a little bit starts blowing warm with no change in air blower pressure. If you turn the car off, when you turn it back on it blows a/c cold again for a while. Time for blowing cold varies. Any thoughts as to what to look at next? Any help would be appreciated.