I have a 1996 F350 w/ 7.3L and I am having a very similar problem. Going along in 3rd/4th and then bogs down to 15-20 MPH. If you deal with it for a couple of minutes it will catch and fire like normal then run great. The bogged down periods became more frequent and long. Now it has quit and will not restart? I was wondering what you found with yours as it sounded similar...?