GPS signal jumps while recording the track

Hello,
Since I started using the track log option in LocusMaps, mainly to keep track of elevation gain and distance to destination while navigating on a pre-prepared gpx track, the GPS keeps jumping down and up, suddenly adding hundreds of metres to the elevation gain.
Please have a look at my screens.
I don't know how to get rid of it. It's just random. I've tried changing the recording values and filtering the altitude, but nothing really helps.
Can anyone tell me what's causing this and how to fix it?

Second thing - I thought maybe an external GPS would fix the problem so I bought a Garmin Glo 2 unit and while it doesn't have these altitude jumps it has other issues such as
- The GPS speed is all over the place, from 5 to 60 km/h, and changes drastically every time it refreshes,
- it lags - what I mean by that is that when using external GPS my position on the map is at least 20-30m or more behind my real position. with internal GPS it also lags, but only a little - like 10m or so.
Since the Garmin Glo 2 refreshes the GPS position 10 times per second, is it possible to set the recording parameters to take advantage of such a high refresh rate of the GPS signal? For example, in the options, the lowest setting of the refresh time is 1 second, so how can I set it to 0.1 seconds?

BTW, I'm riding a bike so no crazy speeds, just about ~20km/h.

Please help :)