The flow of air decreases when ac is running for a period of time. The ac compressor was just replaced.

That is because the pressure in the AC system is either getting too high or too low and the pressure switch cuts it off till the pressure drops to a normal. During that time, the air gets hot and then the switch comes back closed, the compressor comes on and the process starts over. You need a manifold pressure gage set to diagnose.

check that the fan for the condenser is working
with the vehicle moving the air flow will keep the compressor pressure down but when stationary and no air flow the gas pressure will go too high and the high pressure switch will stop the compressor
have it inspected by an accredited ac specialist shop

the ac system needs air through the condenser ( radiator bit in front of the radiator) to operate correctly. IT it has no air then the compressed gas is to hot and will not turn to liquid and so the high pressure switch turns of the ac compressor. No problem when you are moving as road speed provides the air flow. Get an electric fan and have it installed before the condenser and connect it to the ac compressor circuit so that it is running all the time you have the ac compressor running. not the local mechanic.When dealing with ac systems the best people to take it to is an accredited ac specialist shop

this will be from the head pressure being excessive as the gas is not being converted to liquid in the condenser. The main reason being that there is insufficient air passing through the condenser from the ac fan not working. There has to be an extra fan in front of the condenser to maintain air flow at all times the ac is operating. The second reason is excessive amount of gas in the system. Have the pressures checked by an accredited ac shop.

check your ac belts and pulleys. if there ok then possibly your ac compressor. your compressor not only holds freon but also oil it could be low and its burning your compressor

Sounds to me like the A/C clutch is no good and out of adjustment.You should goto your nerrest junkyard and get a used assembly take the clutch off then you learn how it comes off and install it on yours the save the pump for later use if needed. The clutch should only have a .010 air gap after install between th inner elctro magnet and the clutch plate it's self.

IDLE AIR CONTROL VALVE IDLE SPEED TOO LOW. IDLE SPEED SHOULD BE 750 - 850 RPM. IDLE SPEED NEED TO BE CORRECT OR IT WILL CAUSE ENGINE RUN ROUGH OR STALL OUT.AC COMPRESSOR WILL TAKE SOME POWER FROM ENGINE WHEN AC IS OPERATING.

I would check the following possibilities:
1. Is compressor clutch working? - You need to do this when the ac is blowing hot air only - locate compressor drive pulley on engine - rubber belt wraps around it. Start engine and set to max ac. When the ac is blowing hot air, look at the center of the compressor drive pulley using a good flash light - watch it for a while, it should be spinning with the pulley - this means the compressor is engaged. If not, the compressor is not engaged. If your ac is blowing hot air, then the compressor should be engaged and running (trying to cool). If it is not engaged, then your compressor clutch is probably going out. Could be low voltage to clutch, but more likely the clutch is worn out and only working some of the time. Note: if working normally, the compressor clutch will engage and disengage in cycles- but the air should always be cold. If clutch is bad, compressor would need to be replaced (sold as one unit). This is tricky to diagnose and could take some time - you need to do it when ac is blowing hot air but set to max cooling.

2. Low on refrigerant or refrigerant has moisture in it? If your ac blows cold initially after being off for a while, then heats up, and continues to regularly cycle back and forth between hot and cool air, then the system may be low on refrigerant or the refrigerant may have moisture in it that is freezing and restricting refrigerant flow. An a/c shop can check refrigerant level and even replace refrigerant if moisture is suspected.

Could be other issues, but I would start with these.

Good luck

does the AC compressor run under the hood? if not u have lost the charge out of the system and u have a leak..

if you have the filter missing you a/c evaporTOR MIGHT GET DURT AND decrease the air flow, but the cooling should be ok
is it blowing hot air? or the air flow is low?