Dell G15 5520 120Hz Display: Heat-Triggered Horizontal & Vertical Lines, Flickering, Ghosting

My Dell G15 5520 (15.6”, 120Hz, 1920x1080) screen glitches when it heats up. Need help diagnosing!

Symptoms:

• Cold boot: Screen is fine.
• After heating up (5-10 mins in 30-35°C, 30-50+ mins in cooler conditions):
  • Horizontal Lines: Start with gaps, then every other pixel (1 pixel on, 1 pixel black) across the screen. Small text unreadable.
  • Flickering & Ghosting: Early flickering; ghosting when switching apps (e.g., app A lingers).
  • Vertical Lines: Colored vertical lines appear mid-issue, persist after prolonged use. Flickering/ghosting stop when horizontal lines “max out.”
• Refresh Rate: Lines start at 120Hz. Switching to 60Hz clears them for 2-3 mins, then they return. After hours at 120Hz, 60Hz doesn’t help.

Details:

• OS: Linux, latest NVIDIA/Intel drivers, BIOS updated.
• Tests: LCD BIST (Power + D) shows lines. External HDMI monitor has no lines (GPU likely fine). Reseated display cable, cleaned vents, used cooling pad—no fix.
• Suspect: Display cable? LCD panel? Motherboard?

Questions:

1. Cable, panel, or chip issue? How to confirm?
2. Known G15 5520 heat/120Hz display issues?