Hi,

I can say it's not good to let gasolines to age in a cars gas tank! There is a lot of air above the gasoline in a 16 gallon tank that will cause the fuel to oxidize or breakdown more quickly and the octane drops down with this aging because gas is not a wine!
You can slow down the rate of decomposition if the tank was completely topped off when it was stored, in a garage, that keeps the temperature more constant.
The same thing should be done with your gas cans, if they are used storing fuel for the same amount of time.

So, You have new spark plugs in there and this engine is in a good tune up condition.
Did you do the replacement of parts or did someone else do it?
Do you know if their gap is set correctly?
Some people are told by the parts counter clerks, that they come set from the factory in its little box if you are lucky. Bubble packs can get dropped from a hook and stuff found dropped on them to add insult to a possible injury.

I never believe that they are going to follow quality control that close to each box! One heat range of a spark plug can go into too many different kinds of compression engines, using different ignition voltage methods, timing control algorithms and so forth.
Each car maker can use different fuel minimum octane ratings.
Volvo calls for a 89 minimum octane rating in any red block.
OLD gas can be the issue for a while if the tank was left near empty.

A misfire, even a slight one, will be picked up by the rpm drop.
The misfire results in a quick mixture changes from the ECU and that affects the oxygen sensor to respond so it can be called a self-tuning fuel management system.

What is the age of the oxygen sensor?

Does your car use a only slight amounts of engine oil? High oil consumption creates more carbon deposits.

Look at the colors on each plug to be the same across all of the cylinders?

Of course, this is my armchair recommendation!

Phil