When you say "Retro A/C replaced about 5 years ago", was it converted from R12 (freon) to R134? You are probably low on whatever refrigerant the system contains and/or you may have a bad heat control valve that is allowing hot engine coolant to "dilute" your cold A/C discharge air. How often you need to recharge A/C systems depends on how big a leak you have. There is a marker dye that is injected into the system to detect leaks. FWIW if you do have the R12 system, the cost of freon has gone through the roof now that it is no longer used and it goes up daily. It's reached the point that it is often cheaper to convert a R12 to R134 system than to pay for a couple pounds of freon.