Russel, if it was only a couple of tanks you should be OK. My concerns would be that an '85 model was probably not compatible with unleaded fuel and thus the valves and valve seats may have taken a bit of strain. The other thing is that if you're running on a fuel octane rating that is too low, you also run the risk of burning valves. Pull the plugs and examine them, they will give you a clue as well if the electrodes have been excessively 'eroded'. I would replace with new plugs at the same time and see if that makes a difference.
(With my older model I used to keep a couple little bottles of lead substitute additive behind the seat in case I had a situation where leaded fuel was not available and I had to fill unleaded.)
If your problem persists after you have looked at all the possible alternative causes (fuelling, ignition etc.) then you might have to pull the head off and have the valves checked. If it's not too bad they can be re-ground, but it would be in your interests to replace them with valves and seats that are compatible with unleaded fuel which will save a lot of aggravation down the line.
. Have put a few tanks of e10 e85 when fuel prices raised . At times when going around corners . Or inversed upwards long hills car wants too cut out or runs on like 3 cylinders . have been back on unleaded . Will fill with premium fuel next time . Fuel filter not old , but but yes start again . Suggestions fuel tank other issues I have likely too have caused . 
