Question for auto transmission gurus.

[evo2]

My '01 4.4i 135,000 mi X5 shudders during light load acceleration.
It will not shudder much at all under hard acceleration.

I though it could be a worn torque converter or trans fluid having over 100,000mi.
After much debate and reading all the horror stories of changing auto trans fluid, I decided I would change the fluid.
(dropping the pan and replacing the fluid/filter, not a power flush)
Figuring the viscosity was too low in the old fluid and causing some slipping.

However, one day I put the trans in manual to gear down for a big hill and it didn’t shudder at all.
I’ve been driving it for a while now in manual and it drives fine.

Does anyone know what would be different in manual mode vs. auto what would prevent the shuddering?