Wi-Fi Drops After iMac Enters Sleep Mode

Please help. I've had a lot of Wi-Fi issues after upgrading to Yosemite. I know a lot of others have had similar issues and the Genius bar hasn't been able to come up with a solution. I get the same "I've never heard of this problem before" and then run the simple diagnostics. Here are some specs:

- iMac 21.5" 2011 desktop

- OS X Yosemite 10.10.5

- Have had off and on issues with Wi-Fi connectivity problems depending on the current version, but the worst problems have come with Yosemite

- iPad 2, Asus laptop, and HP printer work fine. I've tested the Modem and Router, they are working well also

The problem: after waking my iMac up from Sleep Mode my network connectivity is dropped. I have to physically click on the Wi-Fi connection and wait for it to find the signal (takes about 30 seconds). It's not a large issue, but it gets very frustrating after it happens every time. I have other users in the household who also have this problem and they're not very good with iMacs.

Has anyone had this problem and been able to fix it?

Thank you in advance,

Scott