By drum, do you mean piston?If there's pressure loss/delayed/no shift when cold but it shifts when up to temp, it's most likely a hardened bonded drum seal.
It's very unlikely to be the valve body, in fact from what I've been seeing most of the P2714 codes in the A760 and AB60 transmission seems to be seal hardening and not the valve body as typically gets blamed.
Running 210+ degrees for 150K+ miles (like these transmission with a thermostat/heaTer and no auxiliary cooler tend to run) is really hard on internal seals.