Really easy internet connection question?

[Andy]

So I pay for a 10mbs download speed from Cogeco Cable, and most of the time that is what I get. However, at times I can be around 1mbs while my brother's computer only 7 metres away is still at 10mbs.

The problem must obviously be that my wireless connection to the router is screwing up my internet speeds when something interferes with the signal. My brother with the older and worse computer has a consistent 10mbs download speed all the time- because he is hooked up to the router.
Also, It can't be my router, I recently bought a 750mbs Dual-band D-Link- it's great.

My question is, is my only option to get rid of the wireless connection and set up a connection to the router? If so, how do I go about doing that?

Also, why are computers sold with a wireless router connection when it's so inferior to a cable connection?