Donna you should try to drive this car out on the open road and run it hard sometimes. I have a '95 E320 that I drive on weekends (minimum 70 mph - 30 miles without stopping) and I notice when I first start it up after having sat all week long that the initial idle is slightly rough. I will probably start driving it a little more than once a week. In addition to being hard to start these cars do some weird things when the voltage starts to fall off.
How long do you go before you have to put more fuel in it? Gasoline can kinda go bad over a period of a year or so and can possible "gum up" the fuel system. The Techron should help this some.
a) Try to run it more at higher rpm - This will keep the battery charged.
b) Do the OVP relay as suggested.
c) Change the oil more than once/year.
d) Go back to original spark plugs.
The worse thing you can do to this car is drive it 3 miles, shut it down and then do it again after it has cooled down. Believe it or not this is considered "severe service" in the manual.
When was the last time you changed your coolant, brake and transmission fluid?