I have found a trouble in determining the cause of Atmega168 restart. According to the documentation, the 3rd bit of the MCUSR register should be 1 if MCU restarted due WDT (00001000b). But seems that this bit is never 1. Checked other bits - PowerUp bit (00000001b) is working correctly.
Wasnt able to find anything about this in errata...
Hope someone have solved this somehow and can help with the issue.