I doubt he was having a bad day, because the events I mentioned above happened over 2 months, many visits to the shop (at least 10 visits in those 2 months period), and every visit took on average a couple of hours!
The reason I am not mentioning his name is that may be he is a good mechanic and that's why others have recommended him. He is also a very nice person and I don't want to affect his business--that's how he makes his living, you know. May be it's just that he had bad constant bad "luck" with diagnosing my car, I don't know.
Also, on many of the visits he didn't seem to be in a rush: he had no cars in the garage, he was on the cell phone, took his time to inspect the car (with the frequent cell phone breaks), and charged me nothing--so he wasn't in a hurry to make money, you know.