1994 s-10 blazzer loose power util 20 mph

I have a 94 Blazer, 4dr. 4.3 "W" motor with CMFI intake (http://www.automotiveforums.com/#)Ideals rough and has no power until i get to 20 MPH than it kicks in and i have full moter the fule filter and pump are new just had a tune up replaced the Plugs, Cap, Rotor, Wire Set ful tune up when it is not in gear sounds fine when i hit the gass when it is in netural or park sound like the moter is all there but when i go to drive its sputters and runs rough with no power until 20 mop or so an iedeas i rally need help on this one. thank you