I didn't experience SOD on the .471 either but my experience from other devices are as follows when it comes to narrow the problem down:
1. A rogue app. If the device suddenly starts to SOD after a couple of installations, then there's an app causing it to hang when entering deep sleep.
2. Excessive undervolting (not relevant for us yet). One SOD cause is too low voltage to the CPU, causing it to lock up when it enters deep sleep. The problem can also be connected with underclocking, for example to 100 or 192 MHz.
3. Hardware failure.
4. ROM problem. There are ROMs, even official ones that are prone to SOD.
The solution is to make a full wipe and then just wait and see what happens with the device when it is completely clean. If it still suffers from SOD even if nothing is configured, then it points to a hardware problem or ROM bug. Trying a different ROM can be worth a try.
If that still doesn't solve the problem, i.e. it continues to SOD regardless of ROMs - there is a hardware failure.
If it works fine after a full wipe but then starts to experience SODs when apps are installed etc, then there is a rogue app involved.
My longest uptime on the 471 was around 5 days and 6 hours before I updated to 475. No problems during that time.
Xperia Z Ultra, Xperia Z, Xperia X1, P910i, HTC HD2, Nokia E90 et al.