Dear All (Especially electrical engineers),
 
A couple of years ago I got a "partially deffective gtx 670 FTW" for cheap and hoped for a repair.
Before I start going into detail please note - I know 670 isn't worth anything but I'm doing it for experience.
Symptoms: Some polygons randomly disappearing for a single frame. 
Direct cause: Under load GPU voltage fluctuates wildly.
This is where I got stuck many years ago. Since finally I got myself something I can call a lab (half a room is already something :) ) I decided to dive deeper.
This model uses a soldered-on RT8802A module which (thanks god) is well-documented.
After many hours of tooling around and looking at things another fact was discovered: When load appears V_GPU right on GPU goes down sharply but PWM is slow to respond.
First thing I noticed when comparing to reference schematic - values of feedback filter circuit are not exactly as in reference diagram. While one resistor is 15K as must be second is 18K instead of 15K.
Could not measure capacitors because I'm afraid I won't be able to solder them back properly.
The picture shows voltage on PWM (right after inductors) on top and feedback voltage after 15K resistor on the bottom.
As you can see the PWM side goes up when feedback goes down but not fast enough. 
Tried playing with nominals - only got more noisy output.
If anyone has an idea what could be the culprit - please don't hesitate.
What I already tried:
Added some more tantalum capacitors on the back of GPU - zero change.
Measured +12V voltage drop on MOSFET sources - under 250mV.
Connected wire from V_GPU to PWM module feedback input for possible bad track in pcb - no change.