Intermittent picture and sound break-up Sony Bravia Android.

[Windblown]

We just bought a Sony Bravia 55" 4K TV. We are mostly satisfied with its performance, but have concerns with its behaviour when viewing via digital aerial in the 'TV' app. 

Most of the time we get really good reception and good  image and sound,  even on HD channels. We assume, therefore, that there is no problem with signal strength. Then, for no apparent reason, mostly when changing channel or just after switching on the TV, the picture breaks up and sound stutters in SD and HD. Waiting a few minutes, changing channel or restarting the TV fixes the problem. This is obviously unsatisfactory. We want a TV that works like a regular TV using a digital aerial signal when we want it to. 

Any ideas of what might be causing this or suggestions for fixing it?

Our set up includes a symmetrical 1Gb Ethernet connection and a 1TB SSD USB drive for recording TV programmes. Downloaded apps include Plex, Kodi, Express VPN (running only for Kodi) and  VLC. Firmware is up-to-date.