Ford Super Duty (1999-2016) :: Occasional Misfire With Low RPM - High Torque

On my '99's 5.4l with 55k, I've got a slight misfire occasionally with lower rpms and higher torque - basically going up a grade that's not quite steep enough for the truck to downshift, but clears up right away the instant it downshifts and gets some rpm. It generally seems like weak spark, or perhaps an injector spray pattern such that it doesn't always get fired off.

To give some history, the truck had more or less sat in a farm garage for ~5 years driven maybe once a quarter before I picked it up ~2 years ago. When I first picked it up, it misfired all over the place. I didn't think much of it, considered it gummy fuel, and dumped a couple cans of SeaFoam in the tank, replaced all 8 plugs (pretty sure with motocraft plugs though I don't explicitly remember), checked my air filter, etc and the misfires cleared right up and it purred nice and smooth like a happy little motor. That was at ~45k, or 10,000 miles ago.

Now it's starting to hiccup again and I'm not entirely sure why. The plugs should be nowhere near burned up yet and there are no lights on the dash. It's never come close to overheating and the only oil leak I know of is that famous small drip on the inside-rear of the passenger side bank drips on the exhaust header just enough to stink every so often when I come to a stop. I don't believe I'm burning any oil and certainly not near enough oil to foul a plug. Including that drip, I go about 3,000 miles before needing to add a quart. I've dumped 2 bottles of Berryman's in the fuel tank and it seems to work, but it's definitely not gone, so I'm thinking it's probably not oil related and more fuel/spark.

I'd had it in the back of my head to replace the fuel filter as well, but I hadn't gotten around it yet as it seems to be getting fuel at high rpm's just fine. I went ahead and bought the filter the other day and plan to install it shortly. It doesn't really make sense that that's what it would be, but I figure I'm due and it's worth a shot. If they were old plugs, I'd just replace them out, but they seem to new to be causing any issues.