Possibly a faulty injector with a leak off circuit fault but have also had problems with fuel line o rings that have shrunk letting air in. Have a close look at the fuel lines when the engine is running for air bubbles- if air can get in then the fuel can run back to the tank.
If we get a cdi in nowadays with a fuel pressure/
starting fault we always start by replacing the
o rings even if there is no sign of air in the clear plastic lines- they only cost pennies &
never get changed until the car breaks down.