Blows warm air

this may be a goofy question BUT do you mean flushed the radiator not the heater core. Heater core is on the firewall.
Check the hoses going into the heater core to be sure they are both hot after the car has reached running temp. This will let you know if you have a blocked heater core not radiator.
Just wondering?

you will need a thermometer, you can use a meat therm.take a temp reading of the air going into the return and then coming out of the supply at the unit, it should be at least 15 to 20 degrees cooler in the supply, if not it may be low on refrigerant also check filter and return duct make sure your not pulling air from attic

Does the furnace stays off?.
Furnace are ment to cycle on and off because of limits .If you set to 70 furnace would cycle several times before reaching that temperature.
Just make sure your filters are clean and you got proper return air flow because lack of air flow will cause furnace to shut off sooner than normal

2000 Nissan Altima Heater Air Condition Section HA 76 pages Here is factory service repair manual for your vehicle . Do you know how to do testing on these system ? Automotive electrical system testing ? How to read a wiring diagram ? When hot outside is the inlet pipe to evaporator freezing up ? Do you know where this is , under the hood in the engine compartment at the fire wall . What are gauge readings , Is the compressor engaging ? Were any switches replaced ? Ambient temp. , thermal transmitter switch . pressure etc...

Clean the outside and inside coils and filter. This could be an air flow problem

The pressure changes with temperature, it is not a true gauge to ensure a proper charge.

You should have a 15 to 20 degree temp drop across the evap coil. (return air vs supply air)

Can you verify that the compressor is running?
Not the condenser, but the compressor located inside the condenser (outdoor unit)

it really sounds like it is still undercharged.
Check the temp drop, let me know what it is.
Also tell me what the outdoor ambient temp is.

( for example.... 98 degree outdoors with 85 degree indoor temp could result in pressures something like 325 on the high side and 75 or 80 on the low side.
With the exact same unit, but 80 degree outdoor temp combined with 70 degree indoor temp, your pressures could read 215 low side and 55 low side. Not to mention that more efficient units often have higher pressures on the low side because they have more coil space and remove heat more efficiently)

undersized is just that...you and I got taken to the cleaners.but i did put an adjustable vent in the cod air return vent duct next to the floor...it helped mine but i say some installers should.....well you know

PROB NEEDS 2 JUST BE RECHARGED. BUT STAY AWAY FROM THE DEALER. IT IS ALWAYS 3 TIMES MORE THAN A PRIVATE SHOP

I assume this is a heat pump. The outside unit may be icing up. It should have a defrost cycle with electronic control that sometimes fails or sensor associated with the electronic determination of when to defrost may have failed or may not be sensing because its out of position. You can check for a defrost problem. If frost is blocking the outside unit air will not pass through it after the frost builds up and you may be able to see the white frost as well. If all this is true you need to get a service manual and use its procedures to find out why the unit is not defrosting.

That's crazy, we're having the same problem.  Turns out there was a wattage restrictor in the furnace that was switching off 3 of the 5 heating elements in an attempt to save energy.  Basically, the restrictor was sensing that the incoming air was warm, so only 2 of the 5 coils should be needed to heat the air.  Well that wasn't enough and we just sat with the furnace running all day long struggling to reach the 70 degree setpoint. 
This is something a technician can diagnose in a few minutes, but not the sort of thing a DIY'er should really handle.