Engine troubles??

[BIGTEN]

Truck was running fine couple weeks ago and all of a sudden it barely stays running, I have to keep my foot on the pedal to keep it goin and when you let off it shakes and sputters and is extremely quiet. I changed all the vacuum lines to the carb but still the same, did the starting fluid trick to see if it got sucked into anything but nothing. This is extremely baffling?? I took it out the other day to see if I had fixed anything and went down the street it wont go past 35 and the auto trans stays in gear for about a minute and then shifts over. I can have the pedal to the floor and its like it doesn't want to move, but it does if that makes any sense. Its a 350 sb all stock with all the smog setup with a q-jet 350 trans if that helps. Also when you move the gas pedal in the right spot just a touch the carb sounds like a sucking vacuum dont know if this is normal or its just the engine getting its air?