Cossack, my first response is, D-Link is the worst company in the world. I owned a 605(Worst N router ever made!, 615 (I bought a second D-Link, I am a glutton for punishment!), 635 (I smashed it to pieces)
http://www.youtube.com/watch?v=imTxtPY4N1o
Ok, Cossack, do you connect your PC to dir 300 with wire or with wireless?
Supposedly D-Link put out new firmware for all the DIR3XX models that is supposed to fix some of this. So maybe flash to new Firmware.
Secondly, this is a usual problem for wireless if there is interference....
With your router turned off, look at all available networks. Try to find a channel (1, 6, or 11 in the USA, I do not know what channels Latvia uses, sorry, ) that is not in use or has the weakest signals. Set your router to that channel.