Transmission malfunction warning light BMW X5 2009 possibly faulty?

Had the transmission malfunction warning light come on a total of four times in about 5 weeks. It will only come on if I'm on the highway going 80 miles per hour or more for 15 to 25 minutes. If I pull over take the key out and put it back in the car runs normal. I took it to BMW service center and they could not find anything wrong with it. No codes came up for them. They reset the transmission computer. I change the transmission fluid filter in pan after the first time the light came on. It did run better. The mechanic said there was no metal shavings when he changed the fluid so it looked good. Nobody can figure out why this light is coming on and every mechanic says my transmission is good. One of the mechanics a code came up ratio monitoring clutch C. But like I said BMW did not see anything wrong with it and no codes on their end. Of course it happened again a week ago and I don't know how to fix this