It may be worth removing the Mechatronic unit and testing the E-clutch port with air pressure to see if the rear stator bush is worn. The E-clutch is used in 4th, 5th & 6th gears and the proportion of input torque it sees is 72.6% in 4th, 125.5% in 5th and 100% in 6th. It therefore tends to slip first in 5th gear, but eventually the transmission refuses to upshift beyond 3rd gear when the controller detects slip in all three upper gears. If this were the cause, the transmission would slip less when cold, as the fluid viscosity is higher and therefore the leakage past the bush will be less.

Removing the Mechatronic unit also gives the opportunity to check the condition of the bridge seal and the four jump tubes.

Phil