What can be the problem and how can I fix this car?

My car keep on cutting off, no one seem to know the problem. I have already change my alternator, carburettor, fuel pump and fuel filter and also fix the radiator but to no avail. When it cut off, it would sometimes make a boiling sound. That recently stopped. After the first cut off my vehicle would sometimes start right away, then after a few cut offs, swinging the key, it would be like it doesn't have any fuel and will keep on swinging with no start. Also after a few cut offs it would then mess with the power and give me a hard start. If it cut-off on a hill, it's a nightmare to restart and will cut-off again too numerous to mention. Most time when it cut-off I would allow it to take a 20mins rest and then start back to give me a longer distance in travelling before cutting off again. I'd just realize that I have a huge knock under my tank. What can be my problem and how can I fix this car...