05 4.8is Transmission GA6HP26Z Snap-On Code 4F8A

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Hey All -

As the title would indicate, I've got a transmission fault. I've scoured these and other forums but not found my exact symptoms. Any help would be greatly appreciated.

The X5 is a 2005 E53 4.8is, Trans model GA6HP26Z. Just under 60k on the odometer.

The problem originally occurred when starting off from a red light. I was facing up-hill and making a left hand turn through the intersection. There was a car coming at me and I needed to give a little extra on the go pedal. I launched away as normal but then, sssslllliiiiipppp.... I let off the throttle just in time to hear a chime and see the notorious TRANS.FAILSAFE MODE appear on the display. The transmission completed the shift and I drove away as normal. The display cleared itself after shutting the car down and restarting.

I researched the issue and decided to start by verifying the oil level. (not leaking)
I drained the transmission into a clear jug. The oil was not black, did not show any signs of "glitter" and did not smell burnt.
I refilled the trans (following the instructions found on this forum) and according to my measurements it took a full liter more to fill it than what I was able to collect while draining it.

The X drove perfectly after that for 4 days. Then, exactly the same thing happened under the same circumstances. Facing uphill, turning left, hard acceleration. It got me thinking - so I tried again. This time from a dead stop on level ground, wheels pointed straight ahead. Pedal to the floor = same results.

The only computer I have access to is a Snap-On. The code registered is:
4F8A Ratio Monitoring gear shift 4-5. I am unsure why it would be monitoring the 4-5 shift because it works beautifully there. The problem I have is either 1-2 or 2-3 ***EDIT: Problem is 4th to 5th gear, upshift only***.

I'm planning to pull the pan check the valve body next.

Should be noted that this thing downshifts like a dream, does not suffer the known lurch issue, and if driven "conservatively" one would not know that this problem exists. But you can bet your ass I didn't go for the big V8 so I could drive around conservatively...

Anyone have any input?