If you have good fuel without any air to the injector pump, I would check the Injector Pump return line check valve. It is located on top of the Injector pump and has a rubber line with a clamp. Remove the rubber hose, unscrew the check valve. Look up inside the check valve. The check ball is clear so you should be able to see through it in bright light. If it is dirty your injector pump is probably failing. You can clean it and try it. You may get some more life out of it. but best not to run it in that condition. This is an easy check it should only take about 5 minutes.