LG G Pad V400 downgrade problem

Hello. My tablet ( V400 LG G Pad 7.0) is a little bit slower on original ROM (Android 5) so I tried to install a custom rom (CM). I rooted the tablet, and when I tried to install another .img file using a software, the tablet is remain in secure boot problem or something like that and the only way that bring the tablet back is restore using LG Software. Ok, so I tried to downgrade to Android Kit Kat (4.4 original ROM). I did everything using LGFlashTool2014 but the tablet is looping in LG logo, restarting over and over again and I have to go back at 5.0 original ROM. Can someone help me to install CM ? Any ideeas?

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