Strange Wi-Fi problems, tried different access points

[vl1]

My story is: I had a DLINK DI-524 for three years, which died 3 days ago. I had some minor issues with it and while choosing a new one, i thought no DLINK. Also, friends' advised Dlink beacame shity lately.
So i went for Asus. WL-520gC it was. And here it started.
With the default firmware i started loosing WI-FI. A ping from a notebook connected via Wi-Fi had lost lotsa packets. Like 10 pass, then a loss, then maybe 2 pass, a loss, then 40 pass and two losses. Plus slow speed - like 400-600kb/s, while Dlink worked on 2200-2400kb/s speed. and the most interesting the network could just disappear. Neither the notebook, nor the Nokia phone are detecting the network for like 20 seconds and then it reappears again.
I tried some non-official firmware's, Like Oleg's formware from russian asus forum. It is more stable, still same wi-fi problems. Then there was a DD-WRT firmware, which provided with a superior wi-fi productivity, no packet lossess, great speed, BUT it could just loose the internet connection (I have PPPoE) and never restore it PLUS i had same ping problems, but with the wired network. It worked VERY unstable.
Ok, i go back to the shop, and give the ASUS back, and for the same money i choose 3COM, 3CRWER100-75-ME model. Oh, 3COM, i thought, the best from the best )))) Ha, ha - same problems, but NO wireless network disappearence. the network could be found all the time, but same ping problems, and low speeds on file transmission. They are better then Asus though, may vary from like 600-700 to 1200-1400 kbp/s, but still i have no 2mb/s.
Here i start to understand, that it was not Asus who was faulty, but there is something around the room where the AP is located that gives some interference.
I have 3 PC's on wired connection to the router, plus 2 laptops and 2 nokias over wi-fi.
All the cables and the internet modem are gathered in one room, so moving the AP to another romm is unfortunately not an option. Nevertheless, i disconnected all the cables and took it to the other room, where i connected one laptop with a network cable and another one via wi-fi. Even less ping drops, and the speed goes up to 1800-1900 kbp/s.
So, i am even more sure, that there is something that prevents the wi-fi from working normally with maximum productivity.
I tried everything - disconnecting all Tv cable in the flat, unplugging Bluetooth modules from the nearby computer, disabling my home's DECT phone, disabling all the 3g mobiles, turning off monitors, computers themself, i even unplugged two power back-ups that i have (i thought those UPS could make some interference with their electrical nodes).
NOTHING! Still same packet losses and low speed!
I tried all the Wi-Fi channels. From 1 to 13. Nothing. I tried different AP transmission power rates. Still nothing. And tried different #com firmwares for this devices, from older to newest unofficial - nothing. same things happen.
Any ideas?
thanx for reading and maybe someone could give me a hint. Cause i really have no desire to give the 3com back ang going again gor the cheap DLINK.

[vl1]

noone?