Strange Gray flashy bar seen only on Windows metro apps

i have been noticing strange gray bars on my windows metro apps such as video,camera.it' sometime on the middle of the upper half,sometime on middle if the lower half(only for metro apps,other parallely running apps work fine without any grey bars,the problem reappears on switching to the metro app).i heard that if your graphics card is faulty the problem wont appear on hdmi connected tv.well i dont know whether it is correct or not but the problem appears on the HDTV also.my alptop is Dell inspiron 15Z 5523 ultrabook bought it in november 2012.thanks in advance for helping
i tried to take a screnshot but the gray bars dont appear in the screenshot