Thank you Logan.
Now we know, that multiple errors with IA7260 Firmware in Event Log are related to LTE modem.
What i also noted, sometimes pair of these error messages happens during time, when device is sleeping, in the middle of night or in the middle of long untouched period of time at home or work. So, there might be one more reason for its crushing.
Answering to Barb's question - yes, i have swapped out my first device, then returned to the Store second one and purchased 3rd one 2 months later. All 3 have LTE and all have this issue, confirmed by many other Customers. I hope MS will finally look it up and fix the problem.
This issue doesn't seem device specific. The company I work for has over 8000 surface 3's and I can replicate this issue on all of them I have checked. Unsure as to the effects so far as they don't have LTE access yet.