Another possiblity I know of besides ignition or fuel system is a plugged exhaust system (front catalytic converter especially). I have seen a few of these, they act kind of like you say, you need to put the throttle down to get it to start, and if you let off the throttle it'll die. If you believe the engine is in a good state of tune, and it didn't start acting up right after getting gas (in which case I wonder if you got a bad tank of gas) I would think about this possibility.
To verify it, loosen up the bolts that connect the exhaust pipes to the exhaust manifold to relieve the exhaust back pressure, then see if it runs normally. Don't leave it like this, because it'll be louder than Hell. And also the possiblity exists that you'll get cabon monoxide into the cabin.