I have a 2013 Chevrolet Cruze LS, 1.8L non-turbo, 6-speed manual transmission.  I purchased the car used from the dealer network that sold it new, and had always serviced it, and it had a complete service history.  It currently has 87,000 miles on it.  I know, not your favorite car, but I like pretty much everything about it, and so far it has been 100% reliable.  The only service it has needed is routine maintenance.  Lately it has developed a moderate issue with intermittent misfires.  This occurs mostly at 2,000 RPM or less under moderate acceleration, such as when maintaining speed when going up a hill, and is a very noticeable jerk/buck.  Dropping down to a lower gear to increase engine speed above 2,000 RPM smooths it out, as does flooring the accelerator.  This only seems to occur when the engine is fully, or nearly fully warmed up.  When cold it runs fine.  Under hard acceleration I think I can detect a slight surging, but it's barely noticeable.  There are currently no codes and the check engine light is not lit, however twice it has generated a lean condition code when idling for an extended period of time.  Of course, since this is a smallish 4-cylinder engine it's more noticeable when the AC is running and adding an extra load.  The idle is smooth unless the AC is putting a load on it.  When the AC is on and the engine is warmed up I can feel an intermittent misfire at idle.  It's definitely not rhythmic like one cylinder is missing consistently.  The problem seemed to start immediately after a gas fill up.  The car had been running perfectly prior to that.  I have tried the following: 

1) Added an alcohol based "dry gas" additive to the fuel, thinking I may have gotten water in my most recent fill-up.  This did improve the condition, but didn't eliminate it, and it got worse again on the next fill-up.  I tried this several times.  I have had water in gas before in other cars, and the problem I'm experiencing feels very similar to that.  

2) Checked for vacuum leaks.  None found.

3) Cleaned the MAF sensor.  No change.

4) Replaced the MAF sensor.  No change.

5) I have tried a few different fuel injection cleaner fuel additives.  All seemed to have some effect in improving the issue, but with all of them the problem came back as soon as I fueled up again and the additive was no longer in the tank.  The best result I had was with a Royal Purple product that *almost* eliminated the problem but, again, once I had gone through that tank of fuel and refilled the tank the problem returned as before.  (FYI, I just picked up some Rislone fuel treatment with P.E.A. and am waiting for the next fill-up to give it a try.)

Any suggestions?  Is there any issue common to this engine that is known to cause this problem?