Same issues with WIFI here. The symptoms appear most visibly as Safari being unable to access any web server (while the same servers are all happily accessed from Chrome or Firefox) - so you might write that off as a pure Safari problem. But it is not: other things also malfunction: images in emails are no longer displayed. Calendars no longer refresh, Contacts no longer refresh, and more.
This all happens after I have let my Macbook go to sleep: leaving my desk for lunch, closing the lid, ... Often (not always) when my MacBook wakes up from sleep, I get the above symptoms: most visibly, Safari accesses to any web site, hang.
I have never been able to find a solution - other than turning wifi off, then turning it on again. That makes Safari resume accessing web pages (and email display images, etc). Again, the most visible is with Safari: after turning wifi off then on, suddenly that page request that was hanging completes.
I thought it could have been something with the upgrade from Mavericks to Yosemite. So I recently did a full clean install (that in conjunction with replacing my HD with an SSD - might as well start afresh). No change. Still the same flaky wifi. That was still 10.10.2
Now I just upgraded to 10.10.3. Still no change, still flaky.
Clearly, Apple does not care about us. Rather than bragging about the cool transparency effects in Yosemite and the cool new icons and the cool new continuity (which BTW I find totally useless beyond the initial curiosity), I wish Apple would actually look under the hood and fix the **** engine!!!
I will try the suggestions on http://osxdaily.com/2014/10/25/fix-wi-fi-problems-os-x-yosemite/#comment-1089537 and see what happens.