Safari / graphics card crashes

Hi,

I have a machine that I've slowly been able to get working really well, apart from this one thing-
fairly regularly a page in Safari will refuse to load with 'Safari web content quit unexpectedly'
Often I can get the page to load if I keep trying, but it's getting annoying.
If anyone can suggest where to look, I'd be grateful, can probably fix it myself...

Actually it looks like I have a problem with 'VDA Decoder failed'
But I can't figure out what to do- is this telling me the built in Intel Graphics 630 isn't working?

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Machine spec
Gigabyte Z390 Aorus Master
Intel i9- 8 core 3.6GHz
64GB RAM
Radeon RX 580
Samsung 1tb 970 Evo NVMe

Software
macOS Big Sur v11.4
OpenCore 0.7.0

Sample of crash log

Process: com.apple.WebKit.WebContent [41721] Path: /System/Library/Frameworks/WebKit.framework/Versions/A/XPCServices/com.apple.WebKit.WebContent.xpc/Contents/MacOS/com.apple.WebKit.WebContent Identifier: com.apple.WebKit.WebContent Version: 16611 (16611.2.7.1.4) Build Info: WebKit2-7611002007001004~7 Code Type: X86-64 (Native) Parent Process: ??? [1] Responsible: Safari [884] User ID: 502 Date/Time: 2021-09-30 12:24:07.693 +1000 OS Version: macOS 11.4 (20F71) Report Version: 12 Anonymous UUID: 971B60EF-05F6-BE9B-A07B-C2B55526CE89 Time Awake Since Boot: 96000 seconds System Integrity Protection: enabled Crashed Thread: 0 Dispatch queue: com.apple.main-thread Exception Type: EXC_CRASH (SIGABRT) Exception Codes: 0x0000000000000000, 0x0000000000000000 Exception Note: EXC_CORPSE_NOTIFY Application Specific Information: dyld3 mode abort() called Bundle controller class: BrowserBundleController Application Specific Signatures: Graphics kernel error: 0x067900fc Thread 0 Crashed:: Dispatch queue: com.apple.main-thread 0 libsystem_kernel.dylib 0x00007fff203d992e __pthread_kill + 10 1 libsystem_pthread.dylib 0x00007fff204085bd pthread_kill + 263 2 libsystem_c.dylib 0x00007fff2035d411 abort + 120 3 libGPUSupportMercury.dylib 0x00007fff462cf096 gpusGenerateCrashLog.cold.1 + 95 4 libGPUSupportMercury.dylib 0x00007fff462c621c gpusGenerateCrashLog + 89 5 com.apple.AMDRadeonX4000GLDriver 0x0000000498833302 gpusKillClientExt + 9 6 libGPUSupportMercury.dylib 0x00007fff462c75e7 gpusSubmitDataBuffers + 164 7 com.apple.AMDRadeonX4000GLDriver 0x000000049880d92c glrATI_Hwl_SubmitPacketsWithToken + 112 8 com.apple.WebCore 0x00007fff3b4e6c6d rx::ContextGL::flush(gl::Context const*) + 17 9 com.apple.WebCore 0x00007fff3950478f WebCore::GraphicsContextGLOpenGL::reshape(int, int) + 7151 10 com.apple.WebCore 0x00007fff3a956104 WebCore::WebGLRenderingContextBase::initializeNewContext() + 2324 11 com.apple.WebCore 0x00007fff3a961629 WebCore::WebGLRenderingContextBase::create(WebCore::CanvasBase&, WebCore::GraphicsContextGLAttributes&, WebCore::GraphicsContextGLWebGLVersion) + 3209