My 850T wagon has recently begun stumbling intermittently between 55-60mph. This after it has warmed up.
Can feel it stumble slightly when just cruising along on flat hwy at 55 or under load (hill) at 55. Not so much at higher speeds.
Can also sometimes feel it when idling at a stoplight.
Does not seem to happen at lower speeds than 55 even when under load (hill).

There are no engine codes or CEL.
Runs fine when cold. Always accelerates very well without problems. Starts great.

My first thought is dirty fuel injector(s), but it seems they would act up at other speeds too.
I dumped some 'fuel injector cleaner' juice in the tank and it did nothing.
And the fuel filter has been changed recently.

Any thoughts?