I doubt it's your built in LAN. I looked at your MB specs is a current enough motherboard that you shouldn't be having these issues. Check that your drivers are up to date. You can disable the onboard network card and add an expansion LAN card if you want. I'm currently running my desktop off a USB dongle WifI it works great and is pretty inexpensive.
Here's the dongle I use:
http://www.canadacomputers.com/product_info.php?cPath=27_1048_1053&item_id=042522
And the speed results I get with my 60/10 plan (better then advertised speed) and on Wifi 5G frequency:
http://www.speedtest.net/my-result/3564213248
Worth a try if all else fails