As my my knowledge, no cooling at idle and good cooling at high speed / rpm indicates the compressor has become old and needs replacement.
As I had mentioned earlier, when you replace compressor/ install new compressor you should:
1) Replace accumulator and orifice tube
OR
2) Replace dryer and expansion valve.
Every system has got one of the above within the system.
Proper flushing of entire system needs to be done to remove all the old compressor metal debris due to wear and tear. Flushing includes condenser as well.
For flushing, nitrogen is best, otherwise air can also be used.
Usually PAG oil for A/C compressores are used. Proper quantity of oil is very important to proper functioning of the compressor. Also, measured (weighed) quantity of refrigerant 134a generally should be added. Vacuuming the system prior to adding refrigerant is also a very important step to get rid of moisture.
As mentioned earlier, it is very important to replace accumulator or dryer with installation of new compressor.