I recently went through the v1.6.80 debacle. I had most of the same issues as others. I.e, lost video, weak signal, etc.
I found this posting on DJI Forum site. It worked for me.
Here it is. I'm pasting from the posting, so forgive the spelling and grammar. It's not mine.
Manual Downgrade Phantom 3 Standard (for FW 1.6.80 issue)
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957 16 2016-05-17 10:53:34
reinzneal Offline

reinzneal
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1#
The Steps :
1. Download the firmware at
DJI Phantom Accessories & Tips - Phantom Help and select the fw that you want .bin file format
2. Insert the file to sd card, outside folder, and must be the only one .bin file format
3. Turn on the RC and the drone without sd card until it connect (RC and Drone)
4. Connect to DJI GO App (to see downgrade status stay at front page of the app)
5. Insert the sd card. Then the downgrade process is starting and rear led will turn to yellow and there will be beep sound from the drone (it is normal). See at DJI GO app, there will be status bar show the downgrade process. If the rear led becomes green it means the process is complete, you also can see the succes notice at DJI GO App.
6. Restart the RC and Drone, connect DJI GO App, check the FW. Finish.
I hope it can help the issue of 1.6.80 firmware for Phantom 3 Standard
NOTE : DO IT WITH YOUR OWN RISK!