80% of the diesel problem is cause by fuel delivery and fuel return line.
10% of the diesel problem is cause by air pocket in fuel line or between injector pump and injectors.
Follow the check list to troubleshoot the problem.
==
Check for after market in-line pre-filter between fuel tank (or firewall) to the injector pump.
clean or replace it
==
Fill new filter with fresh diesel fuel
==
Check primer pump for air leak. A leaky pump will cause air to enter the fuel system.
If the primer pump is old then you should replace it.
==
Pump primer for 2-3 minutes to ensure there is no air pocket in fuel system
==
Check for blockage on fuel line
==
Check for return line blockage
==
Do a diesel purge
use short hoses and fresh diesel in a 2 liters soda bottle for to rule out old fuel,tank vacuum and line/filter blockage.
==
==
If the problem still re-surface then crack the injector line one at a time to let the air pocket out while cranking the engine.
BE AWARE OF HIGH PRESSURE FUEL.
Cover the injector with heavy cloth and use strong protection gloves.
==
Check for fuel return on the last injector.
Are the injectors balanced?
Check the factory specification on the injector pump timing.
Start the engine in neutral (if it's an automatic transmission to rule out neutral safety switch problem.
Check for 12V at each glow plug while the glowing to rule out glow relay problem.
Measure the resistance on each glow plugs.
Please post more information on how you tow you VW.