No one knows what my car's problem is...(1999 mercury cougar v6 2.5L Automatic)?

Okay, I’m getting pretty frustrated. It’s been 4 times I took my car to the mechanic for the same problem. It starts at times (esp. when infront of my mechanic) and the other times it won’t start at all (when my mechanic closes) He hooked up the scanner, but there was no error codes. We tried a few things. We replaced the starter, got a brand new battery (as recommended), replaced the battery terminals because it might have been loose, knocked the starter, Checked all of the fuses to see if something has blown, going into different gears before trying to start it. Then, we tried putting all of the electrical stuff on, lights, a/c, radio, windows -sorta trying to surge it - and then it would start. We noticed that once the engine fan starts to turn on, it means that the engine is about to die. The very last time we go it running, we got to our destination, then we decided to turn our head light off. Originally it was on 2 clicks to the right. Then as soon as I clicked it once to the left, the engine died. This is driving me NUTS!!! I don’t know if I should bring it to another mechanic, an electrician, the dealership??? PLEASE HELP

PS. All of these things happened within the span of 9 days.

I vote for an automotive electrician. Unfortunately, there doesn’t seem to be many good ones around; but, ask around. Someone should know of one.