A 78 SEL has CIS so injector spray pattern is less critical than in an injector set to open as the intake valve opens. As Emmerich said, volume is more important on Continuous Injection.

A CIS car that has set up that long probably has corrosion in the fuel distributor and warm up regulator among other places.

Do this carefully and at least a day ahead: put a lot of penetraing oil on the fuel lines at the fuel distributor. If you get too frisky twisting a steel line connector in an aluminum fuel distributor, you have bigger troubles.

At the fuel distributor where the steel injection lines screw in, you can just crack open the connection. Don't need to unscrew the connection entirely. With the engine idling, what flows out may look like muddy fuel. It is rust and such. You can do this with all the lines in turn to get out "the big pieces". Use a rag to collect fuel.

The inlet from the gas tank on the fuel distributor has a mesh screen in the fitting (depending on FD model) and these can collect lots of trash. The screen can be removed and blown out.

There is also a screen in the warm up regulator that is prone to collecting gunk.

The passages in the FD and WUR and particularly the piston in the FD that moves and allows for greater fuel flow on opening the throttle (your trouble) are pretty small and not too much crap will clog them.

CIS is all about flow and pressure. Clogged passages affects both.

I would start out with an expectation that a thorough cleaning might bring the system back completely, but that it probably won't happen overnight. The alternative that too many take is to start throwing parts at it. Fuel distributor and warm up regulators are really expensive.

Nobody can diagnose CIS on the internet. If your thorough cleaning can't bring it back, you'll have to get a fuel pressure gauge and procedures for how to measure flow and pressure. these are in the manuals. Also an excellent book is by Charles Probst and is the "bible" for Bosch K-jet and other CIS systems.

Have you got a new fuel filter? This is going to have to be done now and after you get the clean up done. Most of the fuel in your system is not burnt, but is cycled back to the gas tank, pump and filter over and over.

Because your car starts and only begins to act up when the throttle is opened I would guess that at a minimum your fuel distributor piston is gummed up. But it could be other stuff as well. It's all interrelated and one dirty part affects performance of all the others

it's like your servo and climate control, when one piece acts up, the whole thing can act haywire.

Techron is good cleaner. You can use more and more concentrated cleaners like toluene, but there are issues with toluene that you should be aware of. Try m-100.cc and search on toluene for some more information.

Sorry this is long, but I have spent a long time sorting out the fuel system on my 6.9 and a few tweaks here and there, particularly in the Warm Up Regulator and a good cleaning have really improved the performance of the car.