hesitation in 4th gear/low rpm

[Fish Man]

Hey everyone. I have done some searching but not quite got this figured out. Feel free to link me to threads if I have missed something. My explorer (Specs below, 136,000 miles) has started a weird hesitation/skipping. Seems to usually be in 4th gear...it is an automatic, but best I can liken it to is when you are driving a manual and are in too high a gear. It will still downshift fine, but will skip and generally act unhappy at lower rpms going up a hill on the highway. Fires up just fine every time, but sometimes idles rough at first. With my loud muffler I have sometimes noticed a weird burbling sound, different from the way it normally sounds. Seems to also have low power during these times. Not showing any codes. I have not done anything about this yet, I know it can be many things, but not sure where to start. Suggestions??

 

[my98nnj]

Plugs, wires, coil, fuel, MAF sensor. There's a good place to start.

 

[drdoom]

Fuel filter.

 

[Fish Man]

sounds good guys, thanks. I know those have all done, but will have to check when and tackle them appropriately!