The A/C in my 2002 Ranger, 3.0L, is working for short periods either cooling fine or not blowing air out the vents. I would assume the evaporator is freezing up and the system needs freon. I put my gauge set on the system and read 100 psi on both the low and high pressure sides and, hence, cannot add freon. So what does this mean? I've been told the compressor is worn out but it will cool at times. Any thoughts on this?