An intermittent beam is not one single fault. First establish whether LightBurn stopped requesting output, the controller changed the request, or the laser system could not maintain it. Do not bypass a lid, flow, temperature or other safety interlock to make the symptom disappear.
Reproduce with a controlled file
Create a small rectangle on one output-enabled layer, use a verified material sample, preview it, and run it under direct supervision. Repeat at a realistic speed. If this file works, compare the failing project layer output, shape Power Scale, device profile and generated code. A hidden, disabled or zero-power object is a project problem, not a failing laser.
Verify the GRBL power scale
For a GRBL laser, LightBurn's S-value maximum and controller $30 must represent the same maximum. A mismatch changes the meaning of every commanded S value. Read the current controller settings before changing anything and use the machine manufacturer's verified value. Also check $32. A normal variable-power GRBL laser profile generally needs laser mode enabled, but controller variants can differ.
With M4 variable power, requested power follows actual motion speed. If a tiny job requests acceleration or speed the machine cannot reach, output can appear weaker than expected. Retest at a realistic speed before concluding that the power supply is cutting out.
Capture evidence
Copy the complete LightBurn Console sequence around the interruption. Note whether the controller reset, entered Alarm or Door state, disconnected, or continued reporting Run. Compare the exact physical point of failure over repeated tests. A repeatable point in the artwork suggests project or code. A random interruption across a known-good file points more strongly to connection, wiring, cooling, interlock or power-system causes.