To diagnose the problem one will have to find whether the problem is a compressor control problem, a refrigeration efficieny problem, or an air mixing or heater problem.
Next time the cooling is poor verify whether the comp is turning. Verify that the heater valve is powered closed. After that the system should have its pressures evaluated.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician