I upgraded to 1.4.2; on 1.3.6 my FP2 was basically fine, although once every couple of weeks I would get an apparently random reboot.
1.4.2 is very unstable. My usage pattern has not changed but now I am getting multiple reboots a day. They occur when I am using the phone most often but in different apps - I can be zooming in google maps, clicking send in FB messenger, selecting a photo in instagram. As a rule I am doing something on the screen though.
There are two additional aspects. About 1 reboot in 6, it's not a 'quick' restart; it does the "optimising android apps" thing you get after an update. This takes maybe 20minutes and is a real problem - especially when it does it when I really need the phone! I know of no way to skip this. About 1 in 4 times, it's not just restarting - it is crashing / screen black / whatever and requiring a battery remove/replace to get it going again.
Finally, I have had a few "system UI has stopped" errors at various points. Plus, I use the default FP2 launcher, with a static photo as wallpaper. On 2 occasions in the 3 days since i upgraded it has somehow lost the photo and reverted to a blue fairphone default wallpaper. this makes me wonder if there's a systemUI issue underlying the reboots as well?
Anyway, all advice appreciated. I generally close a lot of apps so little is running in the background.