I have found another fix for this problem which seems to be working with hardware B1, firmware 2.01EU. The symptoms are pretty much the same as with MicroSun. I have recently switched to a 40M/2M cable connection and there is a cable modem in bridged mode between Internet and DIR-825. I realized that the network speed was not what it should be when connecting through DIR-825 (speed was about 5-25M/2M). There were no problems when using Internet by connecting directly to the cable modem.
I pretty much tried all possible tricks and finally found something which fixed it for me: I went to Advanced --> Advanced Network and set WAN Port Speed to 100Mbps. Using auto or 1000Mbps made the problem persist but for some reason 100Mbps seems to be working - I hope that you with the problem do not have over 100Mbps connections.

I hope this hint helps at least some of you. I must say that I am really unhappy with DIR-825. Quality and price definitely do not match with this product! I am also very sad that whatever the problem is, users have to figure out the fixes by themselves. Update to even worse firmware which usually does not fix the problem, power-off and resetting seem to be the only commonly given official instructions. I have spent an awful lot of time trying to figure out all kinds of problems both with DIR-825 and DWA-160. Resetting and reconfiguring seems to be the common way to go with DIR-825.