To reiterate I only have issues with yahoo mail.  And only on my shiny new Dell.  My HP from 6 years ago gets on it in seconds.  So does my iPhone.  

The question is, do any other websites or applications behave similarly?  If they do, it could be a wifi issue.  If they do not, it could well be a problem with Yahoo -- particularly if you have a relatively limited bandwidth Internet connection.  Given all the adware/popups/junk that Yahoo Mail loads both inside and external to itself, that could be the cause.  It's particularly bad on phone/tablet systems, since you end up having to wait for all the junk to load before you ever see your inbox.

Are you using an email client, or web access?

If the system has McAfee on it, try uninstalling that -- it very often imposes a proxy server that can interfere with some websites.

I’m accessing it through the web interface and I uninstalled McAfee in hopes it would fix the issue a while ago.  No luck.  

Having the same problem.  happens on all browsers even with McAfee turned off.  Can someone elaborate on this Killer wifi issue?  Also happens on All of my new XP{S machines. 1 DT and 2 LT.  

Open Killer Networking center  / Killer control center 

and disable all enhancements

I was able to get to yahoo mail by turning Prioritization Engine off in the Killer wifi app. Works every time.

It worked for a while and I thought it was solved.  Sadly, I'm back to having issues again.  I think I'm washing my hands of Dell.

Thank you! Ever since I got my XPS my yahoo email has been giving me "no internet connection" messages when I did have an internet connection. It was the only website that was giving me problems. Un-checking the Prioritization Engine to "off" in the Killer Intelligence Center (which I hadn't realized was on my computer) instantly fixed the problem.  Thanks again for the information.