Do you have INPA or something to check for errors?

My car is sometimes really hard to start, and sometimes I'm not able to start It at all. Cranks good and all like you describe.

IN my case the problem is the glowplugs. These are a bit time consuming to replace, but It's doable.

Get hold of some kind of scanner or get youself INPA and check for errors. My guess is your glowplugs!