Well it’s certainly worth checking the fluid level, but I’m afraid it sounds like the axial needle roller bearing between the B-clutch hub & C-clutch drum has failed and allowed the input shaft to slide inside the B-clutch drum, splitting the O-ring which seals the A-clutch.
When the fluid is cold the pump can keep up with the leakage. Once it heats up, and the viscosity reduces, the pump can no longer keep up and the pressure on the A-clutch drops, allowing it to slip. The controller senses the slip and immediately puts the transmission into failsafe. Once the fluid cools down, the cycle repeats.
Because the A-clutch is used in all gears except 5th and Reverse, these are the only gears that continue to operate.
Phil