If it is real bad, you might as well replace the connector entirely, they're available for cheap from Wiring Specialties. It is likely just a connector issue if the injectors are newer but still have original injector-connectors with signs of corrosion. While it isn't impossible for the injector to fail, especially being the older pintle-style injector, corrosion on the connector is absolutely an issue. I'd go with a pigtail and just solder it in to the wiring harness.