Hi Karmannski, That's tricky as we've not got a lot to go on! But, by the sounds of it now it's cutting out the suspicion is the pressure relief valve failure that's quite a common one doing the rounds at the moment! Take a look at
by our old pal Daz and see if you can do the same test. Replacing the valve is tricky as there are a few part numbers circulating so if you go to replace check closely the part you're replacing is an exact match (Some aren't exact thread size as a few people I've spoken to recently found out and kindly passed the info onto me). Also, struggling to start on a morning could also be glow plugs, so time to test and perhaps replace: https://www.youtube.com/watch?v=2vtGGp4Jqp8&t=8s