major power loss

[96STROKEME]

woke uo this morning went out jumped in my 96 f-350 5speed started rite up no problem took off went 2miles ran fine the i turned on the highway made it up to about 60 fine held there for about two miles then i started loosing speed bad looked out my mirror and whited smoke like crazy and couldnt keep goin i pulled over it was running really rough so i shut her off and waited a few then start he up and still running rough but i could get it to pick up rpms then it was running better and better so i took off again and it seemed to get its feet back and is now running like nothin happend. the only mods are k&n filter and 5 inch exhaust. any help as to what would cause this would be greatly appreciated thanks