Peugeot HDi: Technical issue, but expensive!!

I have a question for the esteemed forum. I have a Peugeot 306 HDi from 1999 that I need to get running again. It was towed to the workshop a few days ago because it wouldn't start. After the Peugeot dealership failed to fix it, it's now my turn to try.
The following was determined: all 4 fuel injectors are defective! Mechanically, they are okay, but the solenoid coil of the opening valve is swollen, regardless of any signal from the control unit.
After replacing it with 4 used spark plugs, the engine started perfectly. It ran well for about 30 minutes without any problems. The test drive also went excellently. I briefly turned it off to refuel, but then it only started on 3 cylinders. The first spark plug failed, at least in the lower RPM range; above 3000 RPM, everything seemed normal.
After restarting, it was only running on two cylinders. You could say that with each start, one injector was failing, and since I only have four, the options were dwindling. Now, of course, the engine won't start; it sputters a bit on one cylinder. I strongly suspect that if I were to tow the car behind another vehicle and try to get it moving through the neighborhood in second gear, it might start, but it definitely wouldn't run above 3000 rpm... so I'm not even going to try that.
Has anyone else experienced something similar, so that we can exchange thoughts on it? I'm very curious to know why these things are popping out during startup before I have to replace them with four new ones. The nozzle voltage during startup doesn't exceed 78 volts, which is normal. I've already replaced the positive control circuitry (transistor, capacitors) in the control unit.

chris