I haven't read all postings here, but I think I have a/the same problem (now). My SD49225T-HN from Feb. 2018 now (since a few weeks) also boots several times (maybe five times) a day.
How many times a day does this happen to you?
I already use a 12V power supply from the beginning. Until today I used an old firmware version from 2017 all the time. Today I did an update to 2.623.0000000.7.R.R4.2510.9A.NR, Build Date: 2018-11-24, but just now I have the first reboot again.
I`ll keep watching over the next few days/weeks ...