You need to start with the basics. Your fuel supply right from the tanks. Any restriction there will cause the injectors to play up. From there you move to the fuel pressure, make sure that is good. Of course you need to make sure that there is no air in the fuel system. Clear lines are the best way to see this. Connect them to the fuel lines going in and out of the engine. Contaminated fuel could also be the problem with many other things. It could even be a wiring harness that is causing the problem. If you describe the symptoms a little better, then I'm sure the members here will help you try and pin point the problem.
My guess would be WHO are you getting them from, are they new, remans, like the posts above, you need to give more info.