If you car has high mileage, it could be other
problems, such as ignition and fuel injectors.
Check under the rotor to make sure all is well. Difficult to check distributor without an oscilloscope. As for injectors, if they are old, they could develop some gunk and do not move properly. This could lead to a spray that is not fully atomized, or worse, stays open too much at idle and leads to rough idle and poor fuel consumption. Yes, not everything is related to the OVP. If all else fails, think about injectors for high mileage.