We have been on Spark Wireless (Everyday Wireless plan) for around 6 days.

We have clear line of sight from upstairs to a Spark tower 500m away across a valley, in a semi-rural area (view from upstairs window in photo below. with tower circled). Not using an external antenna. Currently modem is downstairs (to hook into house network switches, etc).  In the downstairs location, the modem is behind a window and there are several trees blocking the view of the tower.

Average around 50-60Mbps up and down, though down is highly variable.  Only once saw > 100Mbps (at midnight).  Speed tests done over ethernet.

We have a few internet connectivity issues. Sometimes the internet cuts out for 5 to 10 minutes, seemingly randomly.

It also frequently cuts out for 10 or so seconds at random. E.g. at the circled point below, it froze for about 10 seconds and then resumed later, at a lower speed. Then the upload test failed ("0.00" Mbps).

Speed tests also sometimes completely fail part way through (I guess if the cut out lasts a bit longer), e.g:

We did not have these issues with the (marginal) VDSL we switched from.

Is this typical for Spark Wireless (seems unlikely), or could there be an issue with the modem?

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Could we expect an increase in speed and/or stability by using a directional external antenna located high up with clear view of the Spark tower?

At time of ordering, Spark rated our location as 'Excellent' for fixed wireless.