Mercedes Sprinter 316 CDI

I have a Sprinter 316 CDI that only starts when it wants to. The starter motor doesn't even turn over, and after a while, you hear something click, like something's loosened, and then it starts.
I've been going to Mercedes (ITARSA) countless times, and after they've replaced a ton of parts—a relay, the wiring, the battery, etc.—and after putting the van into their diagnostic computer a million times, I still have the same problem.
They used to blame it on the anti-theft system, but it's not installed anymore.
In short, Mercedes has been giving me the runaround for years and hasn't fixed the problem.
Could you give me an idea of ​​what the problem might be, or recommend a reliable garage in Madrid that doesn't just replace parts, to see if they can fix it once and for all? It
usually starts, but once it's warmed up, I can stop to fill up with diesel and get stuck at the gas station until it decides to start again.
By the way, if I push it a little, it starts right away.

Thanks :(

Hi,

I understand it only fails when it's hot, right? Then it's likely related to the starter motor and its components. When it won't start, try bypassing the starter motor directly, with the ignition switch on and no speed setting. If the starter motor starts turning this way, then you have a problem with the wiring, connections, relay, or the switch. If the starter motor still doesn't work this way, then it's an internal fault. You'll need to check, among other things, the armature (rotor) and field windings for short circuits or open circuits (high resistance). You have to do this when the starter motor is hot. You can do this at home by heating the parts in an oven to at least 80 degrees Celsius and then testing them with a multimeter. I hope this information helps. Regards, Jürgen.