As per the youtube help link provided by acc1444 ,on the youtube video setting, i unchecked the Hardware Accelerator. After that i found there is no restart even after i run multiple Youtube video pages simultaneously. But i have to observe it no. of times, before i can declare that my problem is solved. Anyway thank you very much acc1444 for helping me in overcoming the problem. If it again occurs, then again i will inform the forum.