Wifi problems with MacBook 13 Retina

Hello,

i still have wifi-problems with my MacBook 13 Inch Retina. After using OS X Yosemite in some cases the wifi-connection is shows and won't work at all (can't figure out what are the technical details). The only thing i figure out is that in an clear 5 GHz Network it works every time good. In a 2.4GHz dn mixed (+5GHz) it sometimes drops... and hang.

Give me an idea to fix my Problem.

MAC OS X Yosemite 10.10.2 official