E60 auto "trans over temp" warning causes?

Car is 2007 525xiA (ZF 6HP19) with 185k miles.  Trans fluid and filter changed by me (ZF filter, Pentosin fluid) at 165k, about 1 year ago.  No leaks.  Good shifts, no slipping, etc.  Today I'm bombing it up the road about 80 mph with the AC blasting because it's 90 and humid.  Chime goes off, IP displays red gear with thermometer inside it, and iDrive screen says:

TRANS OVER TEMP, PULL OVER CAREFULLY, STOP VEHICLE, AND SHIFT TO PARK UNTIL WARNING GOES AWAY.  IF IT COMES BACK, SEE YOUR DEALER SERVICE DEPARTMENT IMMEDIATELY

So I pull over, stop, shift to Park, and press the button on the stall to cycle through info.  Red gear goes away for 10 seconds, then comes back.  I take a look for obvious stuff.  No leaks, no debris across grille, no bad smells.  So, key cycle, no warning, drive off, 70 mph max, go another 30 miles and it happens again.  Turn off AC, slow to 65, keep driving.  Clear it with button on stalk, it comes back about 10-15 times, then goes away.  I arrive at destination about 70 miles from home park car for about 5 hours, and roll the dice on driving home.  Ambient about 87 F, all good on 10 minutes of surface streets, hit the highway and go steady 70 mph with the AC on the whole way, issue does not return.

Too late to mess with it tonight, so I'm rolling the dice again tomorrow to work and back.  I will check fluid level tomorrow night.

Suggestions?  Experiences / results with this trans?  Thanks!

How about a pyrometer to check and see if the sending unit is bad? I had one fail in a different ZF automatic, eventually it had the trans temp light on even on a cold start. 

If fluid checks out, I'm gonna assume a sensor. Have you hooked up Carly or INPA yet to see the actual trouble code?

Hell, my E61 w/ the same trans likes to randomly get "stuck" in 1st/2nd after I come to a stop and rev nice and high up to around 7k if I don't let off. Won't even give me a code or error notification. Just waiting for E36 M3 in it to break so I can swap in something nicer.

i've been told that a common failure is the thermostat for the trans cooler.   looks like an easy part to R&R, and there are no lights on so I don't know if a scan would show anything.   so i bought the thermostat and seals ($67 locally all in, better than amazon prime) and I will install tonight.

FWIW, drove to work today with no issues, though i did limit my speed to 75.

Somehow, I forgot about the heater control valve that E36 M3 the bed about one month after adding a bottle of Blue Devil stop-leak to the cooling system about 18 months ago.  When the heat quit being hot, I pulled the HCV and found the internal rubber seals swollen and misshapen, and a sludge of stop-leak goo inside all the passages.  I ran a few gallons of clean distilled water through it and refilled with BMW coolant, and all was well until this trans overtemp issue.

That's the manifold thing the trans cooler and thermostat attach to.  Looks like it got DP creampied by the Smurfs.

That's the old thermostat.  The Smurfs were busy!

And that's the trans cooler.  I got the manifold passages clean with a bottle brush and Simple Green.  Assembled and test drove to check for leaks.  Hooray, no leaks!  Next week I will do a proper cooling system flush and replace the coolant, as I think there's still some Smurf Jizz in the bottom of the radiator.