Can anyone tell me what the REAL issue probably is? Otherwise this looks really nice.
Probably going into safe mode. Could be worn out clutches on the bad end of things or failing temp sensor (internal in the trans) or selector switch. Really need to know the diagnostic codes from the trans computer to tell if it's a $100 fix or a $2000 fix. At that mileage with a "lifetime fluid" my assumption leans toward clutches worn, causing slip, high temps and going to safe mode (3rd gear). Now it just defaults there. Making it closer to a $500 car. Timing guides done on it recently?
I don't know if the shifters are the same, but I had a similar issue on my '91 750. Inside the shifter assembly was a set of sliding contacts that told the computer which gear you have selected. Over time these wore away to the point that it couldn't make any sense of it.
The fix was to disassemble and paint over the traces with clear nail polish to recreate what the switch was supposed to look like.
After I did that, I never had any other problems (with the transmission).
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