Then it is obvious that something on your computer or a Driver/s is the cause of your Vegas Pro instability!

As J-V said, go into the help menu, use the driver update utility to download a new driver.

Thank you for the comments. I've updated my NVIDIA driver and that has improved things somewhat but it still crashes if I try to preview on my second monitor. Some preferences screenshots:

Finally, a screenshot of what happens when I go into preview. The LH monitor just goes black and the preview on the right displays a strange message:

It will play for a while but if I click on the LH monitor the program crashes.

This driver update screen is strange. Why does it say "warning" and "deactivate" under the main GPU? Something is wrong.

Here's what it looks like here (laptop 2 GPU setup).

I've just imported a 4k video file and an mp3 audio file, put them on the timeline and tried to play.

A 4K video in a FHD 29.970 interlaced project.
Reason for using that kind of project?
What is the MediaInfo of that 4K source?

Hi RogerS, I've checked Vegas 18 and there is a message after the 'Warning' (neither has an update screen like your screen shot). This is what it says:

I don't know what the rogue program is but I have tried to deactivate GPU acceleration but this does not seem to be working in V19. It now does not display any preview at all and keeps crashing as soon as I press play.

Another thought on my problem... These may be unconnected but I was working OK with Vegas 18 until around the time of a major Windows 10 update. I don't know if they are connected but it's possible, I suppose.

Hi j-v, this was a simple test. I just stuck with the default project setting and imported a drone file I had shot (Mavic Mini in 4k mode). I usually still render in HD using 50 fps. Here's the Mediainfo:

General
Complete name                            : E:\Videos\Stock Shots\RMS Stock Shots\Richard's Drone Shots\[RSD 073] Langley Staithe - Cross River.MP4
Format                                   : MPEG-4
Format profile                           : JVT
Codec ID                                 : avc1 (avc1/isom)
File size                                : 2.72 GiB
Duration                                 : 3 min 54 s
Overall bit rate mode                    : Variable
Overall bit rate                         : 99.9 Mb/s
Encoded date                             : UTC 2021-08-04 06:53:38
Tagged date                              : UTC 2021-08-04 06:53:38
Comment                                  : DE=None,SN=1SFOHC60AB05HV, Type=Normal, HQ=Normal, Mode=P
xyz                                      : +52.5726+1.4948
xsp                                      : +0.00
ysp                                      : +0.00
zsp                                      : +0.00
fpt                                      : -1.60
fyw                                      : -8.20
frl                                      : -0.10
gpt                                      : +0.00
gyw                                      : +1.10
grl                                      : +0.00

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L5.1
Format settings                          : CABAC / 1 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 1 frame
Format settings, GOP                     : M=1, N=30
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 3 min 54 s
Bit rate mode                            : Variable
Bit rate                                 : 100.0 Mb/s
Width                                    : 3 840 pixels
Height                                   : 2 160 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 29.970 (30000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.402
Stream size                              : 2.72 GiB (100%)
Title                                    : DJI.AVC
Language                                 : English
Encoded date                             : UTC 2021-08-04 06:53:38
Tagged date                              : UTC 2021-08-04 06:53:38
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.709
Codec configuration box                  : avcC

Other
Type                                     : meta
Bit rate mode                            : Variable
Default                                  : No

These may be unconnected but I was working OK with Vegas 18 until around the time of a major Windows 10 update. I don't know if they are connected but it's possible, I suppose.

What build of W 10 do you have now, mine is 19043.1165, the latest and it works excellent with Vegas Pro 10 -19

Windows 10 build 19043.1165, same as yours.

Except to try another sourcefile, I'm out of suggestions for you, also because I'm an Intel user.

Thanks. Perhaps I'll have to write off the money I've spent on upgrades and just continue using Vegas 17. I doubt if Magix will be any help.