RPM Engine - Transmission Flutter Help

New here to Forum but not new to Toyota. My daily driver is an '80 FJ40, and others are an '87 BJ74 and 2010 Tundra.
2010 Tundra Dbl Cab base, 4X4, 5.7L. Purchased new in 2009. 220K miles. I live in middle TN, USA
I travel with tundra during the summer about 1K miles per week with a lot of interstate driving.

So, I need help trying to diagnose an issue with tundra that started mildly about 10K miles ago and is slowly getting worse.
Interstate driving scenario....
Problem occurs when: accelerating normally getting on interstate or transmission downshifting to go up hill running 75 mph
Problem: engine speed has consistent flutter of 200 rpm at about 2 second intervals. You can feel it in the truck almost like the transmission is slipping. Best I can tell is that it primarily occurs in 4th (maybe 5) gear. As soon as the road levels out, then it quits. It does not happen at cruising speed. Only when more load applied.
2 weeks ago I drove it to job site and when I went to leave it went into limp mode with traction control light on, engine light on and 4 lo light flashing. I ended up disconnecting the battery and all lights went off and drove great. ??
Last Saturday check engine light came on. diagnosis is secondary air pump/valve not working correctly. I say ok that might be problem. Toyota dealership replaced both air pumps and valves. Yes that was expensive!
Got truck back Friday and drove to FL yesterday only to have same engine/transmission flutter!! But, check engine light is off....
So......Any thoughts, experiences or known issues that I have not been able to find?
Dealership did a fuel system clean at 200K. I have read that I probably need to change tranny fluid but I have mentioned that several times to dealership and all they say is it should be fine.
Sorry, I didn't mean to write a book

 

220k miles? 9 years old? Time to change the fluid. Start there before spending money chasing something that might not exist.

 

Getting transmission fluid changed is definitely next on the list.

 

ASAP!

 

It's most likely the lock up portion of your torque converter. It only activates in 4th, 5th, and 6th gears. Try driving in manual shift mode and as soon as it starts to happen downshift. Changing out the fluid could help. But before I spent a lot of money on fluid that would be drained if it doesn't fix the problem and thrown away you might want to have the dealer diagnose it.

 

Update: I drove the truck another 1000 miles right after original posting before I could get the oil changed and it has not acted up one time. Go figure..??? I did do a fluid drain and it looked a little dark reddish and not burnt. So I had the dealer do a transmission flush. So far all is still good after another 400 miles today. I will continue to monitor and am hoping to get at least another 80K miles out of the truck.

 

Driving In MANUAL MODE?!! Nobody does that!