New 9850X3D + X870E-P: crashes twice on cold boot, then rock-stable rest of the session, stumped after weeks of diagnostics

Built a new PC about a week ago and it has a very consistent, weird pattern: on cold power-on, it restarts once before the Windows logo shows up, boots into Windows, then crashes/restarts a second time within 5-10 minutes of reaching desktop. After that second restart, it's completely stable for the rest of the session until the next cold boot, when the same two-crash pattern repeats. I rarely even see a visible BSOD, it mostly just restarts. Specs: CPU: Ryzen 7 9850X3D Mobo: MSI PRO X870E-P WiFi, BIOS E7E70AMSI.2A52 RAM: Corsair CMH32GX5M2M6000Z36, 32GB DDR5-6000 (currently JEDEC 4800MT/s, EXPO off) GPU: MSI RTX 5070 Ti Gaming Trio OC PSU: MSI MPG A850GS 850W SSD: Samsung 990 Pro 2TB Bugcheck codes seen (all point to memory corruption, no common driver): 0x1A MEMORY_MANAGEMENT - most frequent, "corrupted PTE detected" 0x1000007E SYSTEM_THREAD_EXCEPTION_NOT_HANDLED 0x139 KERNEL_SECURITY_CHECK_FAILURE - "corruption of a critical data structure" What I've tried/ruled out: MemTest86, 4 full passes, 0 errors - RAM seems fine even at stock JEDEC timings Driver Verifier running 24+ hrs with all 3rd-party drivers flagged, never caught an offending driver across several crashes Temps via HWiNFO - CPU tops out at 80°C under MemTest86 load, ~67°C normal use, zero thermal throttling logged anywhere SSD - healthy SMART, chkdsk found only NTFS metadata corruption (0 bad sectors), looks like fallout from crashes not a cause CPU voltage - Vcore was spiking to 1.376V stock/Auto. Applied Curve Optimizer (-10 all-core negative), dropped peak to ~1.345V — didn't change the pattern at all The "crashes exactly twice on cold boot, then perfectly stable" pattern feels like it should be a clue but I can't place it. Almost seems like a memory training/POST issue rather than a sustained-load issue, since it never happens once the system's been running a while. Anyone seen this specific double crash on cold boot then stable pattern before? Sound like DRAM training/POST rather than a sustained stability issue to anyone with more expertise? Known issues with 9850X3D + X870E-P specifically? Already filed support cases with AMD and MSI in parallel, but wanted to see if this rings a bell for anyone here before I commit to an RMA.