My iphone 5 has a problem with wifi. iOS 8.1.3

WIfi gets disconnected when I open and watch the video on youtube app. For sometime that's going fine but after 1 or 2 video this problem continuously occur without buffering anything in youtube just by touch on play button. other problem is it is asking me again and again password of my wifi router But when I switch off and restart wifi on iphone than it easily connects.