[Q] Why the MTK 6589 USB Vcom port disapear after 2 second? topic

I am trying to upgrade my Canvas HD 20 Android 4.2.1. But, everytime I connect the phone with my pc, the "MTK6589 USB Vcom port" appears for only 2 second in the device manager, then it's gone. And the "Device Disconnect Sound" is played. So, I cant flash the ROM. what's going wrong? I have followed every steps in this tutorial.
http://forum.xda-developers.com/show....php?t=2308431

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I have Tried in different PCs and with different Data Cables. But same problem exists.
I figured that, the connection problem I have described previously is the main cause of this mess. can anyone help me out with this?????