Final Resolution - Evaporator Sensor
Thank you all for all the suggestions and comments. My intermitent A/C problem was finally resolved on Friday. The culprit is the temp evaporator sensor.
I wish I had waited a few more days before I sent my car to the mechanic so I can narrow down the possible root causes to my mechanic. Maybe I could have save a few hours on labor expenses.
'98 C230 Smoke