Odd transmission behavior...

[nikonNUT]

Good evening gents and ladies,
I have a 2010 Yaris with an A/T and 217000 miles on it.I had the tranny flushed at 115000 but as I was getting ready for a long haul to Michigan for vacation I drained the fluid (no pan drop), refilled and drove about 50 miles. I then did another drain and fill and off I went. No issues and got between 35 and 37 miles per gallon headed up and 34 on the nose heading back.Flash forward to today and every once in a while when the car shifts into 4th (overdrive or lock-up or whatever) it will hesitate, rev up a bit and engage but not all the time. My commute it 500 miles a week and I'm still getting 35 MPG while on the weekend running around town I get 30 to 32. So... Whats the deal? Cranky TCC? Trasmission eating itself? Clogged filter? Just kinda want to know what's coming so I can prepare for a service or a new(er) Yaris. Thanks for the diagnosis and advice in advance!

Cheers,
Chris

P.S. The drain and fills were done with factory ATF