Experienced a strange problem today. My S320 (W140) suddenly became very reluctant to upshift. Had to increase revs to over 4000rpm then release throttle before it would change up gear. Now I know it has a system that holds onto each gear until 3000rpm when the engine is cold until it gets above a certain temperature. However the engine was already warmed up, +80 degrees C although it was around 1 degrees C air temp.
After I had stopped and restarted a couple of hours later it ran fine.

What controls the upshift points of the gearbox - it is a 5sp auto non electronic gearbox (722.5)?
Is there anything that might require adjustment / cleaning etc?