I have a similar issue with my 2 month old 159. Sometimes it simply won't start, nothing happens when I push the start button (exept the clicking). I first experienced the problem about two weeks ago. At first it would start if I just took the key out/locked the doors or did some other more or less meaningless procedure. Since then the problem has become increasingly common and I have been able to pinpoint the problem to when the engine is “semi warm”. If I drive it for a while and then leave it for 20-40 minutes it won’t start but if I come back after 1-2 hours it will start without any problems, i.e. cold starting works fine... Hence, I believe it's a connection problem.
Would be nice if it only was a cable connection problem… Do you know where this cable was located? Do you think I could find it and take a look at it myself?
I don't know if your 159 is a petrol one. Mine is: 2.2 JTS. I had a similar issue: sometimes in a journey stop to have a drink, the engine just does not start. Not even the starter engine. Always with engine warm or "semi-warm" as you points out. Sometimes I had to try 20 times, in other occasions I had to wait for more than half an hour,...
There's a service news for it: 55.04.08. There's a cable which goes from the fuses box inside the car (mine is below the steering wheel at the left, but I'm spanish -in Spain-, so I don't know the exact position in yours) to the ECU with low section width. The service news replaces this cable by a thicker one. I can't tell you more details since the dealer didn't give them to me.
I suppose the cars after this service news won't have the same issue.
The service news was done 2 months ago, but no fault recreation since then.
It makes sense, if the starter engine does not even start, it means it has not enough voltage. If you can discard a battery problem, then something is "stealing" some amount of voltage somewhere. The combination "high temperature" - "small cable section" increases the resistance in the cable. The more resistance the more voltage in the cable itself, the less in the starter engine.
Hope this helps.