After I updated my phantom 4, my flight speed is not constant. If I have the bird  p mode and at full throttle the speed will go from 20mph to 4 mph. It seems to be unstable during flight. Any help would loved.

I would do a refresh of the firmware.

I tried refreshing the update. I thought that it fixed the problem flew it around some around the house and it seem to fly good.  This pass weekend I was at the beach and decided to fly it one morning. The drone took off and flew great heading away from me, but after couple minutes of flying I notice that the speed was dropping again. Flying the drone back to me the speed would go from 20mph to 3mph then back to 20 and back to 3.  When I finally got the drone back above me I started to try and land it and it would only move at .4mph coming down. It seemed to take forever to get the drone down from around 200 feet.
I think I am going to try and refresh the firmware again and see what happens .  

Hi Jamie,

I'm going to ask the obvious questions... kind of akin to "is the TV plugged into the wall"?
1) Did you connect to DJI (via the internet) after the update?  If not, your speeds and distance are greatly reduced.
2) Did you have the volume up on your mobile device?  If so, did you hear the beeps telling you that obstacle avoidance in kicking in?  I have been confused many times lately about my P4P suddenly flying very sluggishly, only to realize that Obstacle avoidance was actually doing its job.
3) Did you turn off beginner mode?
4) Do you have a gimbal guard installed?
5) Did you notice what the elevation was in DJIGO4?  (Maybe check your flight records)... could be that your drone thought it was only a few feet off the ground the entire time, so it slowed down.

I realize that much of this wouldn't amount to a drone acting the way you described, but these comments would be good to review (and eliminate) because DJI might ask the same.

Mabou2 thank you for the help. I have the beginner mode off , I do not have a gimbal guard installed< and the elevation seems to be correct. I can take the drone up to 200 feet, at this height there is nothing around for the obstacle avoidance to pick up. I can fly the drone down a straight road for several hundred feet and it will fly pretty good. I can turn it around and fly back down the same road and and it might slow down on the way back.  I am not sure if something is going on with the motors and they are getting hot.  It does seem to me that It slows down because it is more unstable now.