I've been noticing a very slight hesitation/stuttering/fluttering loss of power for the last few months. It was super faint and it made me question if I was imagining it, but today it got worse, and confirmed something wasn't right. It is rhythmic, but inconsistent. It fades in and out.

I'm pretty sure it isn't misfiring. No codes, replaced OEM plugs & wires 7k miles ago, and this only happens in 3rd gear, and is most pronounced when cruising at 2000-2500 RPMs.

Search leads me to believe then it is either my clutch or transmission? Both are areas I'm not super familiar with, but eager to learn.

Clutch was replaced with a napa one 50k miles ago, clutch fluid was replaced again 9k miles ago on a mechanic's recommendation after a full inspection. Is 50k normal for a clutch? I think the first one was original that lasted 250k miles. Is 40k interval for clutch fluid normal? or might it point at abnormal wear? Last time the clutch went it was very loud, no noises this time.

Transmission is original, I don't have record of the transmission fluid being replaced, and I can't remember getting it replaced ever. Would have expected that to be called out in the full inspection mentioned above. Should I start by replacing the transmission fluid? Seems a bit more reasonable of an explanation if only the gears for 3rd are gunked up?

Thank you for any help!