I recently purchased a 91 F-150 with a 300 6 cylinder and a 5 spd manual. It has 180,000 miles and has been sitting since last spring. It runs good when acclerated normally or slowly. If you try to get in a hurry with it, it will sorta buck or cut out. It does it in every gear. I don't think its a tranny problem. It is EFI but it reminds me of a carbureted engine kinda backfiring thru the carb or cutting out as they used to call it. If you hold the pedal down too far it will keep bucking over and over untill you ease up on the pedal and accelerate more slowly. It runs great down the highway and idles smoothly and starts right up even in very cold weather. Any ideas what this could be? I put a couple bottles of extra strength gas dryer in the tank and ran some good fuel injection cleaner in it. Thanks for the help.

That soujnds like a fuel PRESSURE problem. You could change the filter (it's a maintence item anyway) and see if that helps or even clears it up, if not, you will have to have the fuel pressure checked. It could be the pump or the pressure regulator at fault (if the pressure is low).