Well, you can rule out the glow plugs.

The first line of attack would be to do a diesel purge on it and see if you can free up anything stuck in the injectors.

If this fails, you may wish to isolate the cylinder causing the problem by disconnecting the fuel to each cylinder, one at a time.

Once you have identified the cylinder, the least expensive item on the list, and the most likely culprit, is the injector. Remove it and bring it to a "qualified" shop for checking, and, hopefully repair.

You might also consider pulling all the injectors and having them tested, however, with the risks associated with some of the injector shops, this might be a dangerous idea.