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Stumbling 99 Frontier

I have a 1999 Nissan Frontier, 4cy, 5speed. About a week ago, it began stumbling sporadically so my first thought was that maybe a spark plug had fouled. I changed the spark plugs, and the problem persisted. At this point, I begin thinking maybe it’s a fuel issue. I went to Advance and bought Lucas Oil fuel system treatment and added it to the 1/2 tank of gas I had at the time. Going home from Advance it ran great. No stumbling. However, it started again the next day. I ran out that 1/2 tank of fuel and filled up again and it is still doing the same thing. It idles a little rough, and stumbles while going up through the RPM range between shifts. Once I get up to highway speed in 5th gear, it seems to do okay. Should I add more fuel injector cleaner or maybe SeaFoam? Could it possibly be a clogged fuel filter? Any advice would be greatly appreciated!

I would replace the fuel filter and get the fuel pressure tested before I added any other additive.