I have a 2002 ford focus 2.0 zetec motor, what is wrong with it?

It will start and run for a minute or two then dies when it gets warm. I can only Rev it half way before it chocks itself out and dies, plugs wires and coil are good and new. I m a good parts swapper but diagnosis is not my strong suit. Someone I do not trust worked on it before I got it and did the fuel system, pump, filter and injectors. Timing belt is worn but ok I think, I did buy the tools to do it, crank pin and cam tool. Now i just need the belt. Can someone please help me I sank my money into buying the car and only have a bit more to fix it. I don t know if it s the cat or fuel pump was done goofy or what.

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