2002 Ford F250 AC only blows cool air over 2000 rpm

I have replaced both of the needle valves, both low and high pressure switches, it's full of R-134, no moisture drips onto ground and blows cool, not cold air. It only blows cool air at over 2000 rpm anything less and it blows not so cool air. Someone told me it should drip onto the ground but this one does not.