@tomas_knot
Hi,
Thanks, I think now it's clear.
Right after boot I tried multiple times same command, with only few seconds in between;
Results are somehow changing;
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
Binary file /sys/bus/i2c/devices/1-0057/eeprom matches
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
Binary file /sys/bus/i2c/devices/1-0057/eeprom matches
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
Binary file /sys/bus/i2c/devices/1-0057/eeprom matches
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
See how last 3 lines there is suddenly no eeprom available ? So something fishy about i2c bus perhaps.
I then went on to think about connecting oscilloscope to see what's going on and what we have done so far. When thinking about where to connect it, it hit me - we have an external 7" Pi LCD connected to Raspberry Pi Display connector and it has those 2 extra SDA and SCL pins... Could they be reason for this... They are there for A and B model Pi's that don't have these integrated to display-cable and as this is version 3 Pi, they should not be connected as those are routed via display cable. So I disconnected them and after boot so far it has performed flawlessly!
Below are results of the same "test" as above, this time results are all predictable and ok, every time there is an eeprom device on the bus.
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
Binary file /sys/bus/i2c/devices/1-0057/eeprom matches
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
Binary file /sys/bus/i2c/devices/1-0057/eeprom matches
pi@f:~ $ sudo grep M503 /sys/bus/i2c/devices/1-0057/eeprom
Binary file /sys/bus/i2c/devices/1-0057/eeprom matches
So, this was my error, not thinking that connection of LCD could anyway be in part of this... Sorry about that.
But many thanks for your excellent support! For me at least, I think the issue is now solved (fingers crossed...).