Hmm, it seems Yosemite has/had wi-fi issues. Depending on what you read, it is either fixed or not fixed.
This article suggest a Terminal command to correct the problem.
This second article says it's fixed, and
this third article suggests going back to Mavericks!
Go figure. Make sure you have the latest updates for OS X 10.10 and Dropbox. I know Dropbox went through a series of updates to correct issues with Yosemite.