Strange macbook pro wifi problems - selective wifi disconnection

Question

I have 3 macbook pros, two iphones and an apple TV.  My primary macbook pro has some very unusual wifi problems that are hard to pin down.  It frequently disconnects and reconnects itself to the internet constantly throughout the day.  I've been testing it in various ways and today I noticed the strangest anomaly by far. 
While working today, my primary macbook pro could not connect to the internet through any of the 3 browsers I use.  Airport Utility said the internet was down (internet icon was amber, airport icon was green).  However during this disconnection, my p2p application (Soulseek) experienced no disruption of service and continued to download files.  I opened my second macbook pro and strangely enough, the 2nd mac was able to connect to the internet through browsers without issue, and the airport utility on the 2nd macbook pro said the internet was connected.  My Apple TV was connected too as well as my iPhone.
It is often difficult for me to troubleshoot network problems because there are so many variables (ISP, airport assembly, airport base station, OSX, airport utility, modem) but I guess in this particular instance, it seems clear that the problem has something to do with my macbook pro.
My primary macbook has always had wifi problems (and honestly, my last macbook--the one that actually worked today!--had similar issues back when it was my primary laptop).  Last week, my primary macbook pro had the topcase, display and logic board replaced by Apple.  When it came back, it had the same wifi connectivity issues it always had.  Often, a restart will be a very temporary fix, but not always. 
I am at a complete loss for what could be causing these lapses in service.  In fact, right now as I was trying to access these forums, I could not even get my primary macbook to open the Apple Support Site on any browser.  Every other website would open, except Apple Support.  I opened my 2nd laptop and it opened Apple Support just fine.  After 15 minutes, my primary laptop finally opened up an Apple support page in Firefox. 
It is really hard to diagnose the problem.  I find that Airport Utility has been so pared down and streamlined to the point that it is worthless.  It is basically a blank window with 2 application logos inside.  It looks very simple and user friendly until it has a problem, and then it tells me to do something very technical without an explanation for the layman (such as assigning DNS servers or putting the airport in Bridge mode).  I always follow Airport Utility's advice, but I still have odd problems.  I am often confused as to whether I should use Airport Utility or the Network button in System Preferences.  They have the same options, but they're arranged under tabs with different titles.
Anyway, does anyone recognize these macbook pro wifi connectivity problems?  Constant wifi disconnection on one machine (while all others experience no connection problems) coupled with instances of selective disconnection (like browsers not connecting while p2p is able to connect or browser connecting to some sites but not all?)
Thanks for any help or guidance
PRIMARY MACBOOK PRO
2014 Retina Macbook Pro 15" OSX 10.9.4
AirPort Utility 6.3.2
AirPort Express 7.6.4
SECONDARY MACBOOK PRO
2013 non-Retina 15" OSX 10.9.4
Airport Utility 6.3.2
AirPort Express 7.6.4

Answer 1

Same exact issue here. It happened once when I updated to Mavericks 10.9.2. It just happened again last night and I can't seem to fix it.
I tried all sorts of things like deleting the SystemConfiguration folder, PRAM, SMC, DNS flush, etc. Nothing helped. As I recall, the last time it happened it mysteriously fixed itself after a day or two.
Very weird problem. I have been troubleshooting computers my whole life but this one is tough to figure out. Other machines in the house connect up just fine.