Tracks Drop Out and Cubase Freezes

I started my Cubase world with Artist 8.5, but have upgraded to 10.5 Pro.
Due to the learning curve on 10.5, I have used 8.5 to record a song, but have encountered problems (my 8.5 Artist now shows as 8.5 Pro).

I recorded approximately 6 tracks with no problem (Instrument for drum tracks and Audio for guitar), but when I tried to record more tracks, upon pressing the record button, sometimes some of the already recorded tracks did not play, or started to play but then dropped out (ie stopped playing).
Cubase would then freeze and the message “Recording Error: Too many tracks recording” would appear - but I was only trying to record one track.

I tried different ASIO Sample settings including the lowest and the highest but it made no difference.

Also, if I tried to play the recorded tracks without trying to record another, sometimes some of the tracks did not play at the start of the song, or if they did, they would drop out seemingly at random, sometimes coming back, but sometimes with Cubase freezing and stopping altogether.

This of course makes it pretty useless and extremely frustrating at the moment.

My computer is fairly powerful as described in my signature.

Any help most welcome. Thank you.