FANUC aiSV 40/40 SP1241 — Accel-only Intermittent, Gate Driver Suspicion

Been chasing this one for two days and need a sanity check. Customer's αiSV 40/40 (A06B-6117-H105) throws SP1241 on spindle acceleration. Random — maybe 1 in 7 accel events. Power cycle clears it, no other alarms, no pattern in load or speed. Decel and steady-state are clean. I reproduced it on our test rig with matching motor and cable. Full accel rate, fault appears. Slow accel 15%, zero faults in 60+ cycles. So it's absolutely tied to di/dt stress, not a hard short. Static checks all came back normal — motor insulation >100MΩ, cable shield continuity good, motor code param matches nameplate, DC bus ~300V stable, no ripple on scope. IGBT static Vce tests consistent across all six devices. Nothing obvious. Here's where I'm stuck. The fault happens only during the current ramp. My first thought was a cracked solder joint on the current sense resistor network — thermal expansion under load causing intermittent open. But I reflowed the sense path and it didn't change anything. Now I'm leaning toward gate driver degradation. The optocouplers on this unit are original, probably 5+ years old. Under high di/dt, if gate drive timing drifts even slightly, you get shoot-through or incomplete turn-off that the drive interprets as overcurrent. Static IGBT tests won't catch that — the devices test fine at DC but might misbehave under switching stress. I don't have a proper dynamic IGBT tester here. Short of building a dummy inductive load rig, is there a practical bench method to stress-test gate drive timing? Or should I just shotgun the optocouplers and gate resistors and see if it clears? One other thing — the DC bus snubber caps. I know ESR increase can cause voltage overshoot during switching, and the overcurrent comparator might trip on that. But I have no ESR meter that can handle the capacitance values here. Has anyone seen SP1241 specifically caused by snubber cap degradation? Machine is a 5-year VMC, 0i-MF Plus, shop ambient 30°C+. I'm in Dongguan, China, if that matters for voltage conditions or parts sourcing. I really don't want to send this back to the customer with "reduce your accel ramp" as the fix. That's admitting defeat. Anyone dealt with accel-only SP1241 and found the root cause? Cheers, Byron