Hi THIRUKARTHICK,
Are you telling that the system start up normally after 20 minutes and works without any interruption?
Meanwhile I suggest that you clean the air vents with a can of compressed air and place the system on a flat hard surface where there is good air circulation. Also run diagnostics on the hardware components of the system. To run diagnostics restart the system and at the Dell logo tap the F12 key every two seconds. This will start the One-time boot menu. Using arrow keys select ‘Diagnostics’ and hit enter. This will run diagnostics on the hardware components of the system which may take around 5 minutes. Please reply to this post in case of any error message