My 1985 380 SE (w126) with just under 100,000 miles on it was working well yesterday, until at about 70 mph I nudged the accelerator and the car started to vibrate as if the motor was missing or pinging.

I pulled off the hwy and at idle it was missing too, but less. Once I pushed the pedal down to drive off it stumbled quite badly.

My thoughts are:

1. I run the car on 91 octane premium fuel. By accident, I addad about a 1/2 tank of 87 octane that morning to the 1/4 tank of premium already in there, about 100 miles before it acted up. As soon as I pulled of the hwy, I topped up with a 1/2 tank of 94 octane Sunoco. The problem did not go away for the remaining trip home. What does a type 116 3.8 liter North American V8 engine need for octane? The manual says 87, but....?

2. I bought the car recently but have no receipt for any service where the previous owner changed the fuel filter. The pump motor seemed sort of loud a day or two before. I will change it now, bit has anyone ever had a fuel filter plug up? What happens? I will pull the plugs and see if they read lean, but..?

3. It was a rainy night. I guess I will check the distributor, rotor, wires, plugs, etc., but if it were getting wet, would the whole thing not just pack it in? I know my car has a screwy idle control computer, but I doubt that would cause it. I guess I will check it again when it is dry.

Any ideas?