Erratic altitude profile on GPS tracking

Happy New Year to all. I use my phone to track my hikes. I normally use flight mode to conserve battery life and this works well. However I find that if I switch flight mode off, apart from using more battery, the altitude profile becomes wildly erratic. This is is best shown by the accompanying profile where I turned flight mode off half way through the hike, at the summit, to make a call and accidentally left it off for the return route, which was mostly along the same track (through forest).
You can see the difference. This is a consistent finding in many different locations using two different android phones (both Motorola Moto G series) and several different apps (Oziexplorer, ViewRanger, MayMyFitness, Endomondo). If I turn off data and Wifi it is no better. If I turn off High Accuracy in the phone's GPS settings, it is no better. Does anyone know why this occurs and how I can prevent it so I can leave my phone's network on while hiking if I need to for some reason? Thanks for any help.

 

Yes I climbed from 80m up to 560m, as the upward track shows. It's the downward track that has the errors I am asking about. I am not bothered about variations of a few metres, but the incorrect altitude points on the downward track are hundreds of metres out, whereas on the way up they are far more accurate. The only difference is the flight mode being turned off. So it's more than just the expected error of using a phone.