Caddy 1.9TDI won't start, sometimes

[Tegheim]

My Caddy -09 with 1.9TDI has been struggling starting lately.
It's like the battery is totally drained, and it just clicks or tries to turn the starter slooooow.
But the battery is fine. I even bought a brand new one.
I have also replaced the ignition-switch at the far back of where you put the key. Don't know what its called.

Where should I start looking?

I used to have ignition-problems according to my OBDEleven, and it dissapeared changing that part. But still...

When it wants to start, it starts right away, like a brand new car. Next time like there's no power in the battery...

[whelp]

From my experience with similar faults, not on a caddy i might add but still...

check the earth bonding. i had a ropey connection on my old van which caused this. i changed the battery and the starter motor before i realised.

I'm assuming you've checked the alternator already...