Driving in manual mode fixes problem, so what is the cause?

[AusPhil]

Hi friendly people.

I'm having issues with my transmission (after doing a few things I'm pretty sure its the trans).

I was having something 'fluttering' regularly, more obvious at lower speed, for instance accelerating out of a corner. Not "shuddering' as such, I'd say more of a flutter, its not harsh. I thought it might have been belt/pulley related so I changed serpentine belt and pulleys. No change.

I wondered if it was my transmission, so decided to drive in manual mode. And wouldn't you know it, couldn't get it to flutter on a 15km trip around the suburbs (and it definitely would have usually)

So the big question is this. What is taken out of the equation when you drive in manual mode? Is it as likely as simple as a particular solenoid in the transmission? If so I just need to know what it could be, swap it out and its fixed?

I'm hoping its that simple! Any thoughts on what it could be?

Thanks!

[Ric]

What model (& year) car do you have?

[AusPhil]

Sorry its a 3.6 with 5eat. Actually a Tribeca

[Ric]

That narrows it down to a 6 year range. 2008-2014.

[AusPhil]

Thought the 5eats were similar enough function wise it wasn't particularly relevant, but its a 2009