Have you opened the case (both sides) and checked the level of dust on all of the hardware, including the CPU's heatsink? Use a new, small, artists brush and gently brush the dust away while holding a vacuum nozzle nearby to remove. Keep particular attention to the CPU's cooler fins. Make sure you don't get any fans spinning if you hold the vac's nozzle too close.
Surface Pro (2017) vs Surface Pro 4