04' XG350 Engine power loss, stalling, trans shifting problem - all at same time and no codes

I have a 2004 XG350 and what started out as a sudden power loss at highway speed with vehicle shaking - feeling like one or two cylinders not firing and engine temperature sudden rise and then return to normal after couple of minutes, to starting from a stop with significant power loss, engine temp rising, transmission difficulty shifting going up hill and stalling - no check engine light/codes.
Mechanic ran test on transmission speed sensor communicating with throttle - no problem there.
Replaced transmission fluid and topped at correct level with correct fluid - no change.
Dropped exhaust to see if catalytic converter was clogged, and it wasn't.
We are at the point where we think the transmission is bad and is causing the engine power loss, etc.
I have not been able to find much information on these symptoms and hope that somebody has seen these problems and found a solution(s), as I'm sure any of us car owners would like to avoid having to replace a transmission (I replaced the trans in my 04 Mountaineer 2 years ago and already starting to see problems again!).
Thank you!!!!