For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
I have this 820-3437-B board that suddenly (no drops, no water damage) run slowly (with fan spinning high loading os),
but sometimes it starts and work perfectly fine for days at full speed.
Try known good LIO board/flex, as TP too.
Also inspect both connectors for corrosion.
Run ASD when doesn't work correctly, or just monitor temperatures with HWMonitor.