1986 300E--hesitation at idle

I have a 1986 300E with 165,000 miles on it. The vehicle runs wonderfully with one exception: at idle only, the engine seems to have a hesitation to it. The idle does not, however, fluctuate, at any time.

The car has done this since I bought it at 150,000 miles a year ago. I put new spark plugs, a new air filter, and a new set of fuel injectors in the car when I bought it to try to solve the problem. I also cleaned the slide valve. None of these things helped. The cap and rotor and spark plug wires are all in good condition.

I put new spark plugs in again at 160,000 miles. There was a little bit of oil on the plugs put in at 150,000 miles, and the car was using about 1 qt of oil every 2000 miles. The guides did not have any play at all when I put the new seals in, but the old seals were somewhat dry. The car's oil consumption has now dropped to almost nothing.

Because the head gasket is BONE dry all the way around, and because the valve guides on the car had no play when I installed the new seals, my assumption is that the car had a valve job previously.

But the car still had a slight hesitation at idle...

My wife has driven the car for the last month. I had not driven it for almost a whole month, and I drove it yesterday. The idle problem was not present! I came to find out that she has been running 87 and 89 octane gas in the car (she didn't know that the car should have 91 or above).

Could there be a correlation between running 87-89 octane gas in the car and the newfound smoothness at idle?

All I know is that my car is now running better and I am afraid to switch back to 91 octane!

Please advise.

Ed