Q: How do I correct network problems on I-Mac?

HI, I have an I-Mac (Intel Core i5, 2.5 GHz) running OS X Yosemite 10.10.2. I recently changed my router and I am constantly having conection problems. I only have problems on the I-Mac, other devices are alright so I guess it's not my router. I already tried renewing DHCP Lease, deleting network pref files (although I'm not sure if the right ones) and of course resetting all devices. The only way to restore network is running Network Diagnostics. Any advice?

Thank you