I have a Ford Fiesta SE from 2012, and I've had this issue for months that feels like a Damocles sword above my head. Mechanics can't pinpoint it, I'm trying here.
I have a massive torque dip around 1500-1700rpm - sometimes I drive through it and I can feel it without it being much of an issue, sometimes I shift up and end up right in that range and it's killing my acceleration entirely and it takes time to recover unless I shift back up - which has been dangerous at the beginning (now I know, but still). Some other times it's fine (though rarely when the car is already developing significant torque)...
Doesn't happen anywhere else.
I've given up on clearing the check engine light, as it comes on very quickly; reports are more or less this - misfires come and go (sometimes random, sometimes 1, sometimes 4), but the MAF is always there and the fuel leak very often. First time I see battery voltage, might have been when I briefly unplugged the battery recently to reset my radio's BT.
Note that the car has a rough idle and is really acting its weirdest when I am low in fuel (Below quarter, though I haven't been there in a while), but the above still stands when full. I must admit I used to run it to the last drops more often that I should have.