What was the temperature at the time? Was it heading toward it's home point?

Unfortunately it sounds like you have the same issues as MANY others.  Worked great then 1.5 added vibration and the toilet bowl effect.  

http://forum.dji.com/thread-36287-4-1.html post #145 form DJI-Ken confirms there "is an issue with the older version Phantoms"

Please upload your flight data and contact support center about this problem .

Morph1 Posted at 2015-12-22 23:22
Thanks Patrick,
You know my P3 is only 3 weeks old with only 6 flights in total,
this is unacceptab ...

Totally understand your concerns ! For your best benefits , please contact our support center and request a data analysis . The analysis result will decide whether it's under warranty . Thank you !

There is a warranty but refunds are only in the first week if you bought it from DJI.com
Since you were in P-GPS mode it should not have drifted unless it lost satellites. Also, I wouldn't be doing IMU calibrations before each flight, you only need to do it when prompted or updating firmware. Make sure it's on a perfectly level surface before the IMU calibration and a IMU calibration will give you faster startup and the "IMU Warming Up" won't be so long.
As far as the vibrations from 1.5, DJI is working on that and it's only on some of the older Phantoms with the older motors and ESC's Some have fixed it by multiple upgrade/downgrades, IMU, compass,gimbal calibrations, and lowering gains.
We can see exactly what happened during that flight if you can upload the DAT file to Dropbox and I will have the data analyzed.
We want you to have a great experience with your DJI product and not have issues.

Sounds good, at least it will tell us why it started drifting.

Morph1 Posted at 2015-12-23 23:07
I'm sorry but what do you mean by whether it's under warranty ? this drone is now almost 4 weeks o ...

Sorry to have made you misundertood . Please synchronize your flight record and upload the .DAT file . So we can give you a better explanation about the accident .

Have you ever resolve this problem?

Morph1 Posted at 2016-1-25 11:44
Well, there a things that are still unresolved , but it I was able to correct them in some way,
I  ...

Im currently experiencing the same exact problem with what you're dealing with. =(
also could you check your pm, idk if it's working or not.

Morph1 Posted at 2016-1-25 12:56
the pm did not work, this forum is very limited in means of functionality,
you can not even make a ...

@morph1
I remember saw your post getting ignored =x.. my email is jdmwhre14@hotmail.com

Works fine on a mobile for me?

Morph1 Posted at 2016-1-26 07:02
sorry I just checked it is now fixed, you can now post from a mobile device, previously the "type  ...

Did you ever upload your flight record to HealthyDrones? It sounds a lot like there were strong winds at that altitude that the aircraft could not cope with, hence it began to drift away.