So I have a problem with my internet and its preventing me from going on xbox live or playing games online. I have two computers and an xbox 360 that are hooked up to its, usually it is hooked up like the following....

Modem- speedstream 5100 24 Mbps downstream and 1.5 Mbps
router- d-link DI-524 802.11g wireless router. 54 Mbps
wireless adapter- d-link dwl-g520 802.11g. 108Mbps

modem in the attic(3rd floor) which has a 80ft chord that goes to the second floor router. From the router i have one 10 ft ethernet chord that goes to my computer, then a computer on the first floor that has a wireless adapter that feeds off the router. Now this connection is the faster of the two but its disconnects for a matter of seconds every 5 minutes(enough to boot from a game).

Then when i want to game i have to use the 80 ft chord from the modem to the router directly to the computer or xbox 360 on the second floor(no internet on first floor because the router is no longer hooked up). When i use the 80 ft chord the connection stays connected at all times but ive come to realize its very slow.

Now i ran some tests on the upstairs computer using the speedtest.net website.  I ran teh test five times on each thing and took an avrege. First is when the 80 ft chord goes from the modem to the router, then the router to the computer. The second set of numbers is a direct connection of the 80 ft chord to the computer9no router)download and upload is using (kbs)kilobytes per second- ping is to a server 100mi away

Download-Upload-ping                  download-upload-ping
2573-403-42                                 922 -67-344
2580-410-43                                 1052-54-348
2581-410-43                                 1165-56-346
2587-410-43                                 1167-42-311
2575-410-43                                 728 -40-359

Ive heard that my modem and router dont fuction together and that is what is causing th drop but i would like to find out what the problem is and how to fix it. ive coem up with the following possible problems and would like to know what you guys think.

1.The chord is to long and it disrupts the speed, which causing breaks that lead to disconnection
2. modem and router dont fuciton together

was thinking about moving the router up in the attic next to the router, using the 10ft ethernet chord to connect them and buy a good wireless card for my computer on the second floor.