I have a Gigabyte motherboard GA-Z77X-UP5-TH
GA-Z77X-UP5 TH (rev. 1.0) | Motherboard - GIGABYTE

For past 2 weeks it is having lot of issues (I have to keep the machine on) as If i try to restart the same or power off. it gives lot of trouble starting up properly.

I get a Main Bios Corrupt Screen. Machine wont bootup (I have unplug ram, unplug graphics card, remove cmos battery etc. etc.)
I have to try various things to get it started.
Sometimes it starts properly, sometimes takes time.. Not sure what is wrong and where..

Can someone please help me how to proceed and fix this issue ?

1. I have never touched this motherboard, overclocked or reflashed the bios.
2. Its very difficult to enter the bios at the moment, I have to unplug, ram, GPU, remove CMOS battery, try to boot pressing case panel buttons here and there and then it boots.
sometimes quick and sometimes takes lot of time.
3. I dont know what is wrong and where.
4. When it boots and enters bios, when I try to save changes and quit bios, it will not boot again..

Please guide right way to fix the issue
How to update properly, this is the only work PC I have in hand at the moment, and lot of important data on same.