OK - Third night success!! 
So here is the recap for me when I forget in a few months time.
Jelly Pro running Android 8.1. I have had a 100% capture rate for the last 2 hours.
I was getting a reading no problem but then the Bluetooth would basically stop working and I would not get any readings when the screen was off. It turns out there are quite a few places where you need to change “battery saving” permissions. I tried to follow another thread (link at the bottom) but I had troubles finding the exact settings that were documented on the phone. So here is is what I did which is a bit of a repeat from the other thread but hopefully this helps find some of those settings.
Settings → Battery - > three dots on the top right - > Battery Optimization
Then you have to hit the pull down thing and select “All Apps”
You should now see a million apps and system things…
I changed the following to “Not Optimized” - Not sure if I need them all.
- Bluetooth (This is key!)
- BT Tool (I have not idea but whatever)
- Bluetooh MIDI (I like MIDI)
- Network Manager (Not sure - I have to test)
- Network Location (Not sure - I have to test)
- xDrip+ (THis is key!)
Go back up a screen and I set the following:
- Stanby Intelligent Power - OFF
- Battery Saver - OFF
Ok - done with the Battery Settings now go to…
Settings → Smart Assistant → Power Save Manager
I have - “Power save manager” = ON
On White list I have xDrip+ checked
I was missing this before which may have been the cause of my problems. Now go to…
Settings → Smart Assistant → Battery Saving Mode → Battery profiles
Make sure “My profile” is selected. Click on the settings for “My profile”…
It should say:
WLAN - on
Mobile Networks - Off (I do no have a SIM card - if you do this may need to be on)
Bluetooh = ON (the switch is a blue colour to the right)
GPS = ON (I am not sure if I need this but it is on right now)
Schedule for switch profile is off on the previous screen.
Now… Back to Settings I have DuraSpeed off.
Settings → Security & location → Location
Location = ON
Mode = High Accuracy
Click on “App-level permissions”
Make sure xDrip+ is set to ON at the bottom of the list.
Location seems to need to be on for xDrip+ but I am not sure of the best settings. These ones work.
Once I did all those things everything worked. In terms of setting up xdrip+ I got the settings from this thread…
EDIT: still running 100% capture for 2.5 hours and the range from the phone is pretty good. Now having 
