For what it is worth, intermittent AC on past posts has often been the AC relay or Aux water pump. Start checking the least expensive items before doing a whole AC or control unit r+r, in my opinion. Also, past posts talk about how to temporarily bypass the klima relay to check for problems. Good luck!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E