I have a late 99 F 350 7.3 auto and had this problem. I tried changing the fluid with no help. I talked with a mobile mechanic and he told me 99% of the time this problem arises, it is caused by the trans filter falling out of the bottom of the trans. It usually falls 1/4 to 1/2 an inch and loses its prime and starts sucking air. This affects the trans because the torque converter requires pressure and flow to release the converter lock. With no or limited flow, the converter lock drifts on. It can get bad enough to kill the engine in reverse or sometimes even low gear 1. After talking to the mechanic, I dropped my trans pan and the filter fell out with the pan. I bought a new updated filter (making sure the old filter gasket wasn't left up in the port the filter connects into) and had smooth sailing for about 3 months. Then the symptoms started to appear again. I knew what was going on and looked online, found a filter keeper clip and ordered one. I again dropped my trans pan and the filter fell out again. I re-installed the filter and installed the keeper clip and have no issues since the install. I feel that most problems like this are a result of the trans filter becoming disconnected from the trans, rather than electrical or fluid viscosity concerns.

 

Just saw this. I did replace the filter. We'll see if the problem returns this winter.