Note the large number of hits on this subject.
It's a wide spread issue.
Here is what the cause is.
When you get in an OD light throttle drive condition, the EGR opens.
At this point the air/fuel ratio goes very lean "by design intent".
This condition requires the coils to offer higher voltage they "are" designed to give.
If a coil developes shorted turns in it's winding the voltage available drops to a marginal or below level.
This causes a missfire on that cylinder as long as this driving condition exists.
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As soon as you downshift, increase engine RPM or make any throttle up or down change that takes the EGR out of operation the missfire goes away.
It goes because the air/fuel ratio has richened to the point the faulty coil now has enough voltage to fire the cylinder reliabily such as going from OD to third gear.
Most often this coil condition does not set a DTC code because it comes and goes and in not a 'hard' fault. Once the fault conditon in not present, the record of missfires to set a code is cancelled so no code or CEL is set for you to see.
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Finding the cylinder at fault.
Four ways;
1. To sub a known good coil in each position until you clear the missfire.
2. Replace all coils but you always have the possibility of a faulty coil in the replacement group.
3. Use a Scanner with a Trap function to freeze frame the live data while drivieng.
4. Have a dealer do a Stress test on all the coils to pick out any that are below limits.
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Plug replacement:
Sometimes new plugs seem to clear the issue only to return several hundred miles later.
Why;
The new plugs are easier to fire by a 'marginal' coil until their tips begin to errode then the required voltage begins to rise and missfire lightly begins all over again. Yes errosion begins that quick with new plugs.
Bottom line is this kind of issue is quite dynamic and complicated involving several parameters that 'stack up' to cause the end result of studder and missfire.
It can get so bad the faulty coil can send interference back to the computer causing it to stop processing any engine data until it recovers.
This feels like the ignition was tuned off and on at a fast rate, a 'bucking' feeling.
And no, its not the transmission doing any of this.
Good luck.