Classic Mustang Engine Help!?

I currently own a 1981 Fox Mustang which I bought to restore. During which I found a problem that wont go away. It only happens sometimes. When I park after driving awhile, I notice a strong smell of gas and it has a hard time starting back up and stalls out. Sometimes the idle is a little rough. It also happens when the car has less than 1/4 a tank. After refilling starts back up no problem. On some days it runs like a champ (quick starting, no stalling...smooth idle) while on other days it drives like a piece of crap. The engine sounds and is strong when I first start it up after sitting but loses power after a few minutes of driving, (sluggish) and makes soft sputtering sounds when accelerating. I don't get it! I changed spark plugs, engine wiring, adjusted carb, timing belt, fixed vac leaks I don't know what else is wrong. I'd hate to have to get rid of it. It is a pretty rare car where I live and I'd like to see fixed up one day. :( Please help??