Mine flashed towing my fiver up a long hill on a hot day in Idaho almost a year ago to the day. The 4R100 AT does not like heat, the stock tranny gauge only reads hot when it is way, way too late, and the stock tranny cooler is inadequate. My 2 cents: Check your fluid and replace if even only halfway bad, and for future reliability install a good tranny oil cooler and a Magnefine filter in line ahead of it. The clutches in the tranny disintegrate under high heat stress and will clog the cooler without the Magnefine. There are fancier setups for cooling and filtration but the greater cost brings diminishing returns, IMHO. That's what I did and I've been fine since. Parts and info at dieselmann's Store: Transmission
It also could be wires chafing in the steering column. You can tell if it is overheating or a wiring issue by the conditions under which the flashing occurs.