I have a 94 Accord 5spd LX. I have been having some extreme fluctuations of fuel economy. Sometimes really sucky ~26MPG, lower than ever for highway driving and sometimes pretty good, close to maximum efficiency ~30MPG highway. What could cause that?

I have been trying to keep an eye on my oxygen sensor. I had one incident about 3 months ago where my car kept stalling and running really crappy, but never happened again.

Can a O2 sensor be intermittent good then intermittent bad?

Here is my fuel economy for the last 2 months. This is all highway driving.

 

Speed, wind direction, temperature changes, etc. will affect MPG.
If you had a stalling issue a while ago I would look into your ignition system if it has not been checked out since then.

 

I have never seen my (all highway driving) MPG to go consistently go low like this, then consistently high. My lowest MPG is normally 29MPG for highway. For city driving, I get between 24-27MPG, usually around 26-27MPG on average. My highway MPG is usually pretty consistent.

My mechanic said the O2 sensor, but the next day, the car behaved normal, and the issue has not come back since.