I've experienced a "slipping" feeling in the low rpms of 4th gear, around that 45 - 60 mph range.
I've got an '04 F150 5.4L and when I would cruise around in 4th and lightly accelerate, the truck would "shudder" or feel like the transmission was slipping, but I could always gas out of it and the truck would drop to 3rd gear, ceasing to shudder.
No one could figure it out until 1 day, the check engine light came on and the truck started misfiring. 1 new ignition coil later and my 4th gear slippage disappeared.
It's happened 3 times in all in the span of 2 years. When the truck starts to "slip" in 4th gear, my mechanic can find a coil that is bad.
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