how often should this be done ???
i have a 1997 C180 where the AC is starting to act up...

When i start driving (cold motor) every now and then AC wont come on cold, i can swap to heat and it gets hot, but nothing cold when changing to Cold. If i stop the car for for gas or something, then AC all of a sudden is freeezing cold.. i can swap to heat and back and all works... Any idea guys ?? what could cause this...