My Venue 11 Pro 7140 BIOS seems to be corrupted

These are my list of problems

1. Very long Dell logo.

originally it took 5 sec to get windows loaded after dell logo show up. but now it takes about 2 minutes to see windows loading animation. and also my venue does not recognizes efi files sometimes.

2.system freezes everytime i do something to UEFI settings

if i edit uefi boot entry from bios, it freezes for 30 seconds. if i do same thing in windows ex)bcdedit it also freezes for 30 secs. even i change screen brightness in bios it also freezes!!

3.Bios refuses being flashed

So, i tried to resolve this issue with reflashing bios. after launching 7140A15.exe system reboots, and then flashes black screen for 3 seconds and then reboots again. nothing changed. even if i use bios update from one-time boot menu it goes same.

I know best solution is replacing motherboard, but my warranty has expired already. so it will cost a lot to get replacement or i have to use aftermarket 2nd hand motherboard..

but i do believe dell support will provide some help...