I would phone the previous owner and ask where he got the injectors from because it sounds to me like one or more of the injectors have a leaking leakback valve so fuel pressure is reduced in the common fuel rail. Problem is it will cost about £450 to get all four injectors reconditioned as I did. But it did fix my cutout problems. You could do a leakback test on all four injectors, many web sites show how to do this using 4 plastic bottles.
If you look at the injectors you will see the leakback clips. Don't bother to try to take the plastic pipes off these clips as they will leak fuel all over the top of the engine. Go to a breakers yard or a Ford main dealers and purchase new clips then you can make up a leakback test kit. You will need 4 plastic bottles + 4 new leakback clips + about 6 foot of plastic pipe cut into four 18 inch lengths. This is a standard testing kit they use at the garages. They quickly unclip the leakback pipes then clip on the test kit then start the engine and watch for which bottle fills up first then charge you £100 for doing it.