Transmission seems to slip when it has warmed up?

Hello,

Most common issues are low or bad transmission fluid or a blocked filter.
Damaged clutches usually cause burned residue in the transmission oil so when draining pay attention to the condition of the oil and any debris contained within.
A mechanical issue in the clutch bands will usually manifest itself in a slipping gear but not all gears.

A faulty valve body can also cause slipping gears as the it may bypass some of the hydraulic pressure required for proper engagement.

A good guide:

https://www.2carpros.com/articles/automatic-transmission-problems

and

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Cheers, Boris