Connection (DNS?) problems on Inspiron 17R SE 7720 with Centrino Wireless-N 2230

I have a Wi-Fi connection problem on my Dell Inspiron 17R SE 7720 with Intel Centrino Wireless-N 2230. Every time after I log in to Windows I can’t use internet for about 5 to 15 minutes. After that time everything seems almost OK. During this time there’s yellow exclamation mark on my connection icon (saying that there’s no internet connection) just for a few seconds, than it dissapears and apparently internet connection is OK (signal strength is very good the whole time), but during this 5-15 minutes downtime I can’t use internet. Websites by domains don’t respond, neither pings do, but when I ping eg. google.com by its IP it responds quickly, so I thought it’s a DNS issue.

Connection settings are automatic. DNS servers (resolved automatically) of my cable ISP respond OK when I ping them during the downtime. So just to check, I’ve set manually DNS servers to Google public DNS servers (both IPv4 and IPv6). It didn’t help.

I don’t use antivirus software other than Windows Defender. The issue occurs with and without firewall. I have the most recent both BIOS (A11) and Wi-Fi drivers (15.3.1.2 downloaded from Intel site). My system: Windows 7 Home Premium PL 64bit. I keep the system updated with Windows Updates. The issue occurs when Windows manages my connection and also when I have Intel PROSet/Wireless WiFi software installed.
I don’t allow Window to turn off the WiFi card (device’s energy settings). I’ve set Roaming Aggressiveness to 5. Highest and Ad Hoc Channel 802.11 b/g to 11 (I’ve seen such a piece of advice somewhere). Nothing seems to help.

Maybe it’s something with compatibility with the router, by the issue doesn’t occur of any other device in my network (Android phones, Kindle, iPad). I’ve got Cisco EPC3925 from my cable ISP. They didn’t give me the password, so for now I can’t change its settings. Its “about” page reads:

Cisco EPC3925 EuroDocsis 3.0 2-PORT Voice Gateway
Model: Cisco EPC3925
Vendor: Cisco
Hardware Revision: 1.0
Bootloader Revision: 2.3.0_R1
Current Software Revision: epc3925-E15-13-v302r125554-120725c
Firmware Name: epc3925-E15-13-v302r125554-120725c.bin
Firmware Build Time: Jul 31 12:51:23 2012
Cable Modem Status: Operational
Wireless Network: Enable

Cable Modem State:
DOCSIS Downstream Scanning: Completed
DOCSIS Ranging: Completed
DOCSIS DHCP: Completed
DOCSIS TFTP: Completed
DOCSIS Data Reg Complete: Completed
DOCSIS Privacy: Disabled

Downstream Channels:
Power Level: Signal to Noise Ratio:
Channel 1: 5.9 dBmV 42.7 dB
Channel 2: 6.7 dBmV 42.3 dB
Channel 3: 6.4 dBmV 42.5 dB
Channel 4: 6.2 dBmV 42.5 dB
Channel 5: 6.0 dBmV 42.0 dB
Channel 6: 5.5 dBmV 42.5 dB
Channel 7: 0.0 dBmV 0.0 dB
Channel 8: 0.0 dBmV 0.0 dB

Upstream Channels
Power Level:
Channel 1: 52.5 dBmV
Channel 2: 0.0 dBmV
Channel 3: 0.0 dBmV
Channel 4: 0.0 dBmV