WiFi constantly drops on Pro (10.9.4); running diagnostics (just running it) fixes the problem for 1-2 minutes.

My MacBook Pro constantly drops WiFi, at home and elsewhere. Other devices run fine. When I run diagnostics, it defaults to ethernet, I switch it back to wifi, it says Diagnostics cannot fix the problem, I run it again once or twice and it picks wifi back up, two minutes later, I have to repeat the entire process. I've looked at similar threads, and none of the offered solutions work, nor are they recently updated. I'm running 10.9.4 (in fact the problem seems worse since the "upgrade"). I also live in a country without Apple Stores. I'm not using Airport, but since the problem is with the Mac and not other devices, and with the Mac everywhere I go, the router is not the problem, nor is the network.