Diesel Bombers - fluctuating power not sure whats the problem is

hi i have a late 1994 f 350 with the powerstroke and well i have had the truck for awhile and done a few things. it stull runs rough i have no idea whats wrong because i cant get any auto enguinity scan tools to read my computer in the truck. anyways what the truck is doing is it idles rough when up to temp and only making about 10 max psi thats at like 3000 rpm and it has power some times and then other times it will just drop flat on its face really annoying! idk whats wrong it does smoke black a little but only the first time i get on it when its cold then after that its super clean. im not sure if its a sensor or my Hpop or something. if anyone has any ideas how to read the computer or just know what might be wrong please help !! thank you so much !