I had a problem that was very much like yours. i went for two years before i found what was wrong. the cam was bad. i had no lifter noise. the car ran just fine around town even on the highway as long as you did not get in to it. it was a gutless dog and would start to backfire under a heavy load. i got a new cam. just because i wanted a bigger one. and the trouble was gone. but in the two years before the cam change. i changed fuel pumps, fuel lines,carbs, exhaust,distributors,wires. nothing helped. I like your fiero. and am building one using a gm transaxle. i would like to see more pictures. and find out more about the build. thanks and hope this helps. Gary