Could this be related?

I'm having problems with random sites not responding every now and then and working fine after a retry. This is only happening on WiFi, wired I have no problems at all...

I've added the MTU key and tried to detect the effective MTU using the ping method described in the article too, but I can't even get a 100% response with multiple tests on the lowest settings.

Three ping rounds (= 12 pings) will always get me at least one time-out on WiFi.

E.g. [ping www.google.com -f -l 548]

Any suggestions?