Sounds like you've had head (gasket) service, and replaced the injector(s), and are getting more injector codes. If you have a code reader (they're pretty cheap on eBay), you can get more particulars. However, you're getting an injector code with a new injector? I'd then suspect:
- The fuel system upstream of the injector.
- The electronics connecting to the injector.
Hopefully there would have been some basic tune-up work done in conjunction with the head and injector work, but...
- If the fuel filter hasn't been replaced recently, consider replacing it. A clogged filter can starve the injector for enough gas to inject.
- How old is the fuel pump? It clearly is working, but if it's on it's last legs and not pumping like it ought to be, again you can be starved for fuel.
- Have you refilled your gas since the work? It's possible you got batch of suspect gas. You might consider dumping a bottle of Octane boost in to see if things run better (or well enough to burn off the old gas).
- Your electronic ignition system (or distributor if the vehicle is of that era) controls the coordination of fuel, spark, and crankshaft position. Check the wiring to the injectors at both ends to make sure there's not a loose connection or cable patch. Check the fuse and or relay for the fuel pump and EIS.
It's also possible that fuel is a red herring, and you might have a really dirty air filter, or fouled/poorly connected spark plugs.
Hoping this helps you narrow it down!