try

ATIWinflash -unlockrom 0
ATIWinflash -f -p 0 YOURBIOS.ROM


In MSI Afterburner your Voltage reading should be lower.

You can use atiflash to both read and write BIOS to the card.
To store a copy of the BIOS on your GPU to disk,  do the following:

This will save the BIOS on the card to a file named origbios.rom:
atiflash -s 0 origbios.rom  
0 if your card is in slot 0.

Store this file in a safe place for later, when you need to flash the card back to original BIOS.

Open the file in VBE, make the changes described earlier and save the result with a new name, tweaked.rom
Now, you can flash your card with the saved ROM:
atiflash -p 0 tweaked.rom.

Then shutdown your computer and restart and you should be good.
Now, if your computer doesnt boot, then assume you bricked your card.
In that case take a deep breath, flick the switch on the top of the card (you card has dual bios).
Restart the comp.  When up and running, flick the BIOS switch back, then run atiflash -p 0 origbios.rom
to restore card with original BIOS.  Then do your trouble shooting from there.

Have never used Deepcool myself, but by the name it sounds like it should do the job :-)
I have used Gelid GC Extreme in the past and have been happy with the result.

This is what VBE is showing for my BIOS.
Overview tab:

https://www.dropbox.com/s/m3xthfui0d6xgl0/gb_280x_1.png?raw=1

Powerplay tab:

https://www.dropbox.com/s/ptpkdq2yic6e3ch/gb_280x_2.png?raw=1

When gpu core is running at freq up to 1000MHz then the VRM is programmet to apply 1019 mV.  
When core is running at a freq between 1000 and 1100 then the applied voltage is 1050 mV.
The measured voltage may be lower in both cases.
When I was playing with this a couple of years ago I was using a wattmeter to confirm the effect of the lowered voltage.
Another note: If you are struggling with high temperatures, then it may help to reapply the thermal paste on the gpu chip.
On the Gigabyte you only have to unscrew 4 screws to separate the fan assembly from the gpu.  If you are not sure, there are
plenty of good guides on Youtube.
Cheers.

Yes, those figures are good.

In VBE, go to Powerplay tab.
Select appropriate VDDC value under the core clock value.
For example at core clock 1100 I select 1050 in the drop down. This means a voltage value of 1.05V when core is running at 1100MHz.
Save the BIOS file and flash the card.
Works fine with my two GB cards.  Allows me to run the fans at 44% with temp at 70deg in open case when core is at 1100.

Don't open up the GPU. The Gigabyte fans are bad and those cards always will run hot.

If MSI Afterburner doesn't work, use VBE bios editor and change your voltage to like 1.050V.

That would only happen if you under-volt too much. Lower it a bit and check, then do the same again with more lower value, do it till you see that something is crashing out in the driver or in the hash-rate.

If you're using Claymore miner, you can try it's own under-volting command.

For the R9 280X cards, they are less efficient than the newer 480 cards, so it is better to undervolt to increase efficieny. So the tempearture will drop.

Right now those cards get around 16.5-17.5MH/s when mining ETH so its normal

If you card is overheating maybe its inside a computer case with bad ventilation, maybe a fan is dying, maybe the thermal paste needs to be respread.

You can also change your bios and undervolt your card and it'll use less power and less heat.

No don't overclock.

Yes you can. but you will have to use a pool. like anyone else who doesn't have a GPU farm.
I'd recommend ethermine.org for a pool and myetherwallet.com if it's not the wallet you're using already.

EDIT: Oh you're trying to solo mine. you'll have to run a full node then. but I really won't recommend solo mining when the difficulty is 100T. you will never get a single block.

Example: