I have been having hesitation problems with my 1995 SL320 since the new gas started (Ethanol) I am not sure if it is the culprit or just a coincidence.

I have changed , spark plug wires, spark plugs and the fuel filter so far. The car starts hesitating (seems like it is missing) and runs very rough. If I shut off the car and turn it back on it runs fine.

This problem is getting worse. It seems to improve if I run Octane boost in the fuel. But also seems worse when it is raining.

Anyone have any ideas? I have thought about Oxygen sensors, fuel injectors or one of the ignition coils, but I have no idea how to trouble shoot this. I don't want to randomly start replacing some of this stuff without some idea of what the probable problem is.