Network connection slows down then stops consistently.

My network connection will slow down and eventually stop on my mid 2010 iMac. I am running it wired to the modem/router but when I change to wireless, my connection will pick back up but slow down again. The issue will continue if I switch back and forth from wired to wireless. I have an iphone and ipad that are wireless on the same network and they don't seem to be affected.

Thanks.