I had the shaking problem and now it is gone

I did the Techron gasoline additive treatment, one bottle in a full tank, next tank none, tank after that Techron again. Good for awhile, then got a slight shake. This time I dumped a 16oz can of Seafoam in a nearly empty tank and so far after a few non Seafoam tanks all is well.

FYI: Water level in radiator reservoir has not changed over the months.

So…
Does anyone know what I’m fixing? Injectors or spark plugs? Advice? I’m planning to sell it soon, so I don’t want pass on an easily fixed problem.

Thanks in advance.