a/c cycling

1997 Dodge ram van 2500. A/C was working ok except once a month I had to top off with R134. I drained the system and had new hoses built because of rust and wear and a leak. A/C shop installed new hoses and recharged the system. Now as I drive compressor cycles on and off every 30-50 seconds. Also Warm air blows out of ducts then cool air then warm again. Sometimes it stays cold for 20 miles or so and other times only for about a minute. Heater valve is new. A/C shop replaced low side pressure switch with new one and adjusted it to draw down to 10 Hg to try and stop the compressor from cycling to much. High pressure is about 300-350 psi. A/C shop claims freon level is full. Engine cooling system runs very cool. Shop seems to think the metering may be bad. Does anyone know what the problem might be?

1997 Dodge ram van 2500. A/C was working ok except once a month I had to top off with R134. I drained the system and had new hoses built because of rust and wear and a leak. A/C shop installed new hoses and recharged the system. Now as I drive compressor cycles on and off every 30-50 seconds. Also Warm air blows out of ducts then cool air then warm again. Sometimes it stays cold for 20 miles or so and other times only for about a minute. Heater valve is new. A/C shop replaced low side pressure switch with new one and adjusted it to draw down to 10 Hg to try and stop the compressor from cycling to much. High pressure is about 300-350 psi. A/C shop claims freon level is full. Engine cooling system runs very cool. Shop seems to think the metering may be bad. Does anyone know what the problem might be?

I've been out of the business for 14yrs. now, But I just replaced the compressor in my 86 conversion van and it sounds to me they have the system over filled,I installed the recommeded 2lbs and my high pressure is only 240 to 250 and it works just fine. Even with the old R12 refridgerant 250lbs was a ideal pressure.This is my theory; If the system builds up too much pressure the compressor wouldn't circulate the refridgerant properly and cause the low pressure switch to kick the compressor off.
Thats Just My 2 Cents !