ADSL Speed Drops Down With Router

[code] My connection is ADSL with 2Mbps broadband. Using speedtest.net for testing the speed I got almost 2Mbps, that is ok, but this is connected directly to the modem. The problem is when I use the router, when I connect it the speed downs onward until it achieve a stable value of 512kbps, that is 4 times less than my contracted speed. I have tried with four different D-Link routers (DIR-280, DIR-600, DIR-600 N150, DIR-615), four ones give the same problem.D-Link tech support people don't have idea about what is happening just like tech support people from my Internet provider. I think the problem is not the modem because I get 2Mbps from it connected directly (without the router).I do the speed test in an interval of 3 minutes between each test, with just one device connected and only doing the speed test, all programs that use Internet access are closed.

Always is the same, after a time the speed drops down, from 2Mbps until 512kbps. The router has the QoS disabled, I have tried changing the MTU value, I have tried some values 1492 (this is the official from the Internet provider), also I have tried some lower numbers 1400, 1200 but the result is the same. The router has as security only wpa2 and it has a static public IP, the modem is configured as bridge. I have also tried unplugging the phones, connected to the splitter, keeping away the wireless phones, moving away the router from the modem for electronic interference, modifying the WAN speed from auto to 10 and 100, I have disabled the firewall, change the channels and power TX. I do the speed tests using the wireless and wired interfaces.