PLEASE HELP!! ~Unable to install update.zip or reboot recovery

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http://androidforums.com – Rooted Zte Merit z990g. Installed TouchNav Recovery Installed Custom ROM via update.zip Phone kept freezing and shutting off again after kids lost sd card. Downloaded and installed Recovery files Phone began freezing again and shutting down on its own after three weeks. Went to go back to stock so that I could send it in for a replacement however I keep getting Installation aborted, failed signature verification, a.d or whole signature verification not found. I stopped right there and decided to ask for assistance. I've read what seems to be like hundreds of topics but n (General)