Trans fluid ok? Trans filter ok? Enough miles to have worn out clutch
bands? Got the trouble code? Got a manual?

Once worked on a car where a trans cooler heat exchanged failed and
mixed the coolant and trans fluid making a hell of a mess. Cleaned and
flushed several times, replaced trans filter, etc. Trans would work ok
for a minute or two and then slip. Stop, rest a minute and try again,
same thing.

Finally put a pressure gauge on it and found after a minute or two it
lost hydraulic pressure, hence the slipping. Pulled the pan yet again
and looked and the filter was clean. Looked at the pan again and poked
at it and found the paint had completely delaminated from the pan and
was getting sucked up and blocking the filter intake. On resting it
settled back down and looked fine. Removed paint, reinstalled pan,
refilled fluid and it ran fine for years afterward.