I had similar problems on SFOS 3.3.0.16, and it was helpful
- to switch to Device only mode as jollajo said above, and
- to switch off the WLAN, Mobile access point and the internet connection.
SIM card may be online in standby mode. GPS reception seems to be better if as less devices or units of the phone as possible are transmitting. Transmitting signals seem to interfere the GPS receiver, that was my experience. But other people said, this did not help… Maybe you try it and give feedback here…