AC Won't blow cold! Has freon!

Yea, AC works on pressure differential so if you increase RPM the compressor is pumping faster which would overcome an issue like a weak compressor or a blockage, then at idle it would loose the differential and stop cooling as well. Get the gauges on it and check it again at idle after running for a while, then at a fast idle like 1500-2000 RPM. On a properly working system you should see pressures matching with this chart at idle and at higher RPM the low side may be a little lower and high side a little higher, but still near those numbers. Low airflow through the condenser would cause high highside pressures but low side would still be low. This kind of sounds like a weak compressor, but they usually get weak cause they're coming apart inside and you'll find metal flake on the desiccant bag and black film on the insides of the lines or the compressor oil will be black. If that's the case then when the compressor is replaced the system MUST be solvent flushed, the receiver drier replaced, the metering device replaced, and the whole system thoroughly vacuumed for a long period of time (30 min at 20 or more inches) to remove all the solvents. All the lines get blown with solvent and air, the condenser and evap core get blown with solvent and air too. Any remaining metal can kill the new compressor. Hopefully that's not the case.