Signature Verification Failed ~ what do I do now?

Zte z990g (roamer) ROOTED.
Bought the phone in Feb, began having immediate issues. Began going through a complete system reboot on it's own repeatedly every 10-20 minutes thus making it very unstable to push through a call let alone sustain ability to send or receive text or mms. The APN settings would never stay set....needless to say, my "dream" quickly became a nightmare.
I searched online, came across a few forums & asked questions.

I later installed TouchNav Recovery and a deodexed, rooted zip of my android. It seemed to help..
for a short time. However, about a month after bliss the reboots began once more, so I opted back to the source I sought previously and was informed the z990g has a shakey kernel that often panics and causes reboots. The fix (as I was told) install an overclocked kernel. Im now overclocked at 777.and it seems worse than ever. Reboots comstantly now, freezes up, lags are horriable. I could go on, but I won't.

Every time I attempt reinstalling stock rom, I receove an error message:
Installation falied;
Signature verifocation can not be found;
Restore now.

PANIC MODE EXTREME! How do I repair this?

I don't have CWM or TWRP recovery (that I know of) and have no clue how to install either one. I tried ROM manager and it tells me theres nothing there.

Could someone please assist me in this? Im not new to Android, but am to mods, and tho I personally NEVER do or touch anything unless I'm sure of it, I know I didn't make a mistake going step by step instructed in the directions.

I need actual help, from a tech expert. Please? I need a step by step instructional on what to do, how to do it, and any other advice one can possibly share. Thank you in advance.