After some ideas to help fault find.

I have one fritzbox 7590 as wifi, router and dec phone etc (IP is 192.168.2.50). Connected to a separate adsl modem with firewall blocking all external access on ADSL (IP 192.168.2.55). Separate NUC on internal network doing DHCP that allocates IP's, Name servers (an internal IP and second as 9.9.9.9) and Router gateway 192.168.2.56 or 192.168.2.55. Have another fritzbox (192.168.2.56) connected to Starlink power supply unit output acting as Wan router. This Appears to work perfectly except below.

This allows me to change any device gateway to 2talk adsl (slow but adequate backup link in case of failures in starlink etc)) or Starlink (very fast) plus with various hardware to have two of every device (or equivalent) in case of failure. 

Do have adsl failures and power failures here in the middle of no-where.

So problem is Starlink keeps having small outages (couple of seconds irregularly over the day and noticed by streaming music stopping or large pauses in internet connections. We both work from home and these are noticeable to the point of being a pain.

If I swap out the fritz 7590 on 192.168.2.56 and replace with old Huawai B315s setup on same IP then problem goes away. Prefer to use fritz but only after can solve problem.

Suspect DNS issue as this setup worked perfectly when originally installed but has become more noticeable in recent times. (creeping small failure problem)

On PC nslookup shows internal name server ip every time when using adsl gateway.

But when using fritz as gateway with Starlink then no matter what i do nslookup shows 8.8.8.8 even when name servers turned off on .56 fritz. It seems to use 8.8.8.8 no matter what settings i change and i suspect that causes the intermittent.

Open to all ideas to dig further.