Any hard bump landings lately? (euphemistic way of asking if you have crashed)...
Everyone will suggest you do an Adv IMU first.
IMO, To do a "successful" Advanced IMU calibration, try not to get the MC temp warning to appear.
Find a spirit bubble level to get the phantom levelled in all axis.
Make sure phantom has not been flown or switched on in the last hour or so.
Start Phantom Assist and find the Adv IMU screen.
Plug USB into pc and phantom. Power on phantom and do Adv IMU calibrations immediately.
Wait 2 min for green ticks - the faster, the more accurate.
If this fails then it looks like the potentiometer (angle sensor) in roll axis is either burnt or on its way out.
Send back for warranty if you are still covered.