You're not alone. Streaming channels through Plug & Play mode on your Ulti/Ultra Box will have occasional buffering issue, even though your Internet connection is in perfect condition. Intermittent buffering will occur much more often when streaming UHD channels like channel 805/806.
Yes, you have 800Mbps Internet, and you can achieve near 800Mbps speeds when doing speedtest on your own devices.
But when you do Test Connection on your Ultra Box, it shows only 20Mbps even when you're on LAN cable connection (and yes, even when you're not downloading anything on your other devices connected to the home network). For me, sometimes I get 30Mbps or lesser. So what does this suggests here?
You can reach out to Astro customer service and explain everything you want, and eventually they probably will replace a new box for you, the intermittent buffering issue is likely going to persists.
I suspect it's probably Astro is trying to
save cost optimising its overall costs on their server streaming bandwidth. In my opinion, this is one of the ongoing issue that Astro refuse to admit (cuz it's gotta be very shameful to do so). Astro keep encouraging customers to go for Plug & Play, because they can save costs and troubles in the long term, especially when it comes to after sales services on the usual Satellite installation.
Long story short, for a more immediate solution to ensure no such thing as intermittent buffering issues when watching linear TV channels on Astro, I suggest you go for Satellite mode rather than Plug & Play.
If you reach out to the customer service for them to do the necessary arrangements in their system and for you, eventually you may be subjected to penalty charges (if there is ongoing contract) as it would be considered as terminating your Astro Plug & Play service, and then migrate to Satellite as new order (that's when they'll arrange someone to install new Astro dish for you) and your 24 months contract may restart. You can dispute all the way to MCMC if you want, but in my opinion, the overall process is going to be tedious. But if you feel safe by going through their official way, then it's your choice.
By any chance, if you still have access/connection to the Astro satellite dish in your home now, you can perform Restore Factory Settings on your Ultra Box (home>Settings>Reset>Restore Factory Settings>Default PIN 0000), and then select
Satellite on the "Astro Box Installation" page.

If you see "SIM Card Error" message once you change to Satellite mode, try Refresh your smart card first. One of the ways to do it, is at Astro Self Service (
https://selfservice.astro.com.my > Account Summary > Card Status > Refresh). Credentials are the same as the one you login to your Astro Go account.
If you're concerned that you're unable to watch Astro due to bad weather, Ulti and Ultra Box does provide Continuous Viewing feature where you'll be given a choice to continue watching the particular TV channel by temporary switches from satellite to Internet streaming, until you tune to the other channel.
This will only work when the box is connected to the internet. As of point of writing, Continuous Viewing feature is available on SD and HD channels, but not available on UHD channels and radio channels.
If previously you don't have Astro dish installed, and you want it now, it will be much less tedious if you hire own installer/contractor to do it for you, compared to going through Astro customer service. If you don't know who to look for, you can search on Facebook "Astro installer" or something like "pasang Astro" followed by your city name. For example "Astro installer Kuala Lumpur", then you'll see lots of installer contact details there. Make sure not to get confused with Astro installers and Astro sales agents ya.
However, if you are in a situation where you are unable or don't want to connect and/or install Astro dish due to various of reasons, then I feel sorry for you.

My recent experience (the last being last night) : when watching live telecast of EPL matches WITHOUT internet connection (with my router turned off) the satellite transmission is not smooth; every 10-15 seconds or so there is some pixelation issue even though there is no rain at the time of the telecast.
My package is Ultra Box with satellite.