You are either not getting enough flow at lower speeds or do not have enough fan.
Verify the activity of your auxilary cooling fan. For diagnostic purposes by-pass the high pressure refrigerant switch and the low speed ciruit should make the fan work. See if that makes a diiference. (this of course only applies if your fan wasn't on at all). Check the your fuses as this is a common problem - the fuses expand and contract under changes of current flow and the fuse cracks.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician