Well you are running out of something. My first thought is available voltage. That is my first thought because my real first thought was the catalyst. and now that I am thinking twice I still think its your cats. I'll wager you cut out the cat under the car which isn't what plugs. What plugs is the two precats right off the exhaust manifolds; not under the car.
But if you had the whole exhaust off all the way to the manifolds then we go back to my first thought: that you are running out of available voltage. To cause your symptoms I would expect the coil or dist rotor finger. This can be checked on a secondary ignition scope, if you know what your doing. Pulling a plug lead should raise the necessaary voltage to 25-30kv. If the spark line doesn't rise above necessary idle voltage or a little more then either the coil available voltage isn't there or it has found a low voltage path; say through the rotor to ground.