Straight 6 random power loss?

[Joe Z]

I've had the truck and issues here and there for about 6 years now and recently started having some odd power loss. It's a short bed 2wd 250ci truck with the Muncie 4 speed. It's a rolling rebuild with all new plugs, wires, carb rebuild, the whole works. but after having a new factory style exhaust installed I started to notice a slight power loss here and there I thought was just from not running open pipes. Now at random it losses all power and the farther I press the peddle the closer it comes to stalling. But if I let off it slowly climbs back to idle and stays there til' I try and accelerate again. But it doesn't just do it when hot or cold and it doesn't always do it for a long period of time. Is it possible the exhaust is clogged or the cat is bad or am I missing something? Thanks.