One possibility....

By far the most common problem with the 5HP24 (now that the F-clutch piston & A-clutch drum stiffness issues have been addressed by ZF) is the failure of the axial needle roller bearing between the B-clutch hub and the C-clutch drum. This allows the input shaft to move rearwards a few millimetres inside the B clutch drum uncovering an O-ring seal which subsequently splits :







The result is that the A-clutch, which is used in 1st, 2nd, 3rd & 4th gears, loses pressure & slips. This would normally cause the control system to engage limp-home mode but, because this would be 4th gear (which needs the A-clutch), the system has no choice but to engage 5th gear - and it often comes in with a bang.

The 5HP24 was originally designed for an input torque of 420Nm and your 4.6is produces 480Nm so ZF presumably increased the system pressure so that the clutches could withstand the higher torques (though ZF did add an extra friction plate to the D-clutch). It’s possible I suppose that the higher pressures would be more likely to cause the bearing to fail sooner on a 4.6is than a 4.4i.

Transmissions suffering from this problem often drive perfectly when cold because the oil pump can keep up with the leakage from the split seal (and therefore maintain hydraulic pressure on the A-clutch) when the fluid is cold and much more viscous. After around 10-20 minutes of driving, though, the fluid thins to the extent that the leakage reaches the point where the pump can no longer maintain the clutch pressure and the clutch slips.

Phil