Today I have bought SAMSUNG GALAXY TAB A 8.0. As suggested in Www.dji.com. I have installed go app 2.6.0. But not working properly.  Screen goes off and hangs after few minutes. Is there any mistakes?  Or should I buy another device?

This device is not on the DJI list as you said.
The only TAB that DJI recommends was the 705C, here's a comparison of yours and the 705C and you can see that the specs is your are a lot lower then the 705C.
http://www.gsmarena.com/samsung_galaxy_tab_s_8_4_lte-6435.php
http://www.gsmarena.com/samsung_galaxy_tab_a_8_0-7121.php

And here's the list of recommended devices.

iOS Version v2.6.0. Requires iOS 8.0 or later. Compatible with iPhone 5s, iPhone 6, iPhone 6 Plus, iPad Air, iPad Air Wi-Fi + Cellular, iPad mini 2, iPad mini 2 Wi-Fi + Cellular, iPad Air 2, iPad Air 2 Wi-Fi + Cellular, iPad mini 3, and iPad mini 3 Wi-Fi + Cellular. This app is optimized for iPhone 5s, iPhone 6, and iPhone 6 Plus.
Android Version v2.6.0. Requires Android 4.1.2 or later.Compatible with Samsung tabs 705c, Samsung S6, Samsung S5, Samsung NOTE4, Samsung NOTE3, Google Nexus 9, Google Nexus 7 II, Ascend Mate7, Huawei P8 Max, Nubia Z7 mini, Sony Xperia Z3, MI 3, MI PAD, Smartisan T1.

Thanks a lot ken

vertical lines in record. what could be the reason for?

Dear ken on 7th April 2016 dji announced firmware update for phantom 3 professional.  should I update or what? what if not? If at all essential then guide me please.