This has been a problem for weeks, but lately is getting worse and worse. The truck (2004 supercab fx4 5.4 136k miles) will accelerate smoothly up until it shifts then when the RPMs drop back down the engine stumbles until i accelerate or let off the gas completely. This is worse the warmer the ambient air is. If I stand on the gas coming from a stop it will shift fine, although with significantly reduced power. Ive also noticed substantially reduced mileage and flashes a left bank lean code. I will also intermittently get a cyl 8 misfire code.

The truck does have a significant exhaust leak but its post-muffler so im not concerned about that so much.

Im leaning towards o2 sensor or plugged cat but both are expensive enough that I would like yalls input before I start shelling out paychecks in parts.

edited to add that the tranny has been replaced, its only got 20k miles on it

 

my truck did the same thing.
Spark plugs fixed it. A misfire was causing the stumble on mine.

 

you were right. Did the plug change friday morning, 8 for 8 came quietly, stumble is mostly gone! Now to find that pesky coil....