Thanks. It looks most likely the freeze is a kernel bug in the 'nouveau' driver:

Jun 30 09:41:31 My-System kernel: [TTM] Buffer eviction failed
Jun 30 09:44:35 My-System kernel: ------------[ cut here ]------------
Jun 30 09:44:35 My-System kernel: Trying to vfree() bad address (00000000ba0a4082)
Jun 30 09:44:35 My-System kernel: WARNING: CPU: 0 PID: 1933 at mm/vmalloc.c:2237 __vunmap+0x1ec/0x210
Jun 30 09:44:35 My-System kernel: Modules linked in: vmw_vsock_vmci_transport vsock vmw_vmci binfmt_misc edac_mce_amd kvm_amd kvm snd_hda_codec_realtek snd_hda_codec_generic ledtrig_audio nls_iso8859_1 snd_hda_intel snd_intel_dspcfg snd_hda_codec snd_hda_core snd_hwdep snd_pcm crct10dif_pclmul ghash_clmulni_intel snd_seq_midi snd_seq_midi_event snd_rawmidi nouveau snd_seq snd_seq_device snd_timer eeepc_wmi aesni_intel asus_wmi mxm_wmi sparse_keymap ttm crypto_simd joydev input_leds cryptd glue_helper video wmi_bmof snd drm_kms_helper fb_sys_fops syscopyarea sysfillrect sysimgblt soundcore k10temp ccp mac_hid sch_fq_codel parport_pc ppdev lp parport drm dm_crypt ip_tables x_tables autofs4 hid_microsoft ff_memless hid_generic usbhid hid crc32_pclmul igb i2c_piix4 i2c_algo_bit ahci dca libahci wmi gpio_amdpt gpio_generic
Jun 30 09:44:35 My-System kernel: CPU: 0 PID: 1933 Comm: gnome-shell Not tainted 5.4.0-39-generic #43-Ubuntu
Jun 30 09:44:35 My-System kernel: Hardware name: System manufacturer System Product Name/ROG STRIX B350-F GAMING, BIOS 3401 12/04/2017
Jun 30 09:44:35 My-System kernel: RIP: 0010:__vunmap+0x1ec/0x210
Jun 30 09:44:35 My-System kernel: Code: 20 e8 28 cc fc ff eb d2 c3 31 d2 31 f6 48 c7 c7 ff ff ff ff e8 d5 fc ff ff eb bf 48 89 fe 48 c7 c7 d8 f6 37 9d e8 ef 97 e2 ff <0f> 0b eb b4 4c 89 ee 48 c7 c7 00 f7 37 9d e8 dc 97 e2 ff 0f 0b eb
Jun 30 09:44:35 My-System kernel: RSP: 0018:ffffc41903d639b8 EFLAGS: 00010286
Jun 30 09:44:35 My-System kernel: RAX: 0000000000000000 RBX: ffffa0d8fb2b0f00 RCX: 0000000000000006
Jun 30 09:44:35 My-System kernel: RDX: 0000000000000007 RSI: 0000000000000082 RDI: ffffa0d93ec178c0
Jun 30 09:44:35 My-System kernel: RBP: ffffc41903d639e0 R08: 000000000000047a R09: 0000000000000004
Jun 30 09:44:35 My-System kernel: R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000f60
Jun 30 09:44:35 My-System kernel: R13: ffffa0d9055d4f60 R14: 00000126c1557d86 R15: ffffc41903d63b08
Jun 30 09:44:35 My-System kernel: FS:  00007f89abd83cc0(0000) GS:ffffa0d93ec00000(0000) knlGS:0000000000000000
Jun 30 09:44:35 My-System kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Jun 30 09:44:35 My-System kernel: CR2: 000055a5ddc1aff8 CR3: 00000007a9666000 CR4: 00000000003406f0
Jun 30 09:44:35 My-System kernel: Call Trace:
Jun 30 09:44:35 My-System kernel:  vunmap+0x30/0x40
Jun 30 09:44:35 My-System kernel:  nvkm_umem_unmap+0x4e/0x70 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvkm_object_dtor+0xa1/0x110 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvkm_object_del+0x25/0x90 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvkm_ioctl_del+0x3c/0x60 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvkm_ioctl+0xf0/0x190 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvkm_client_ioctl+0x12/0x20 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvif_object_ioctl+0x4e/0x60 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvif_object_fini+0x5a/0x80 [nouveau]
Jun 30 09:44:35 My-System kernel:  nvif_mem_fini+0xe/0x10 [nouveau]
Jun 30 09:44:35 My-System kernel:  nouveau_mem_fini+0x57/0x70 [nouveau]