The crash behaviour seems to depend on the drivers used.
18.4.1 -> 18.6.1 (inclusive) result in a GPU reset (both monitors die, system recovers to a 'Unreal process has stopped working" dialogue).
18.7.1 causes a silent crash where the application simply closes with no error message and no driver reset.
These crashes occur reliably within half an hour of play in the actual game-world (not the menus!!!).
18.3.3 has occasional crashes (in the first variety - gpu reset and monitor losing signal before recovery) but much less often than later drivers. They can be eliminated entirely **but only in 18.3.3** by disabling TDR detection in the Windows Registry. Doing this in later driver versions reduces but DOES NOT eliminate the crashing.
DxDiag attached for reference, though everything is fine here
Thanks, but I have FSO disabled globally in the registry (along with all the other extraneous XBox crap).
I tried essentially every system-display setting I could find before embarking on my survey of every driver AMD has put out since March - up to and including a clean Windows install. Given that I've solved it completely (30+ hours without a crash vs literally 5-30 minutes with other drivers) I'm quite confident where the problem lies here.
Didn't help I'm afraid, though thanks for the suggestion - not entirely clear on how a memory management issue would manifest only under certain sets of drivers, unless those drivers were themselves flawed in some respect, but I've tried essentially everything else I can think so, it was worth a shot.