inquiry 1: after opening it, some white substances are on the boundary of power board. scratchable by bare finger nails.
can mac expert enlighten me if this white substance is an issue and causes my imac frequent auto restart? mac user diagnostic report said no trouble, i know it is not accurate. do i need replace power board?
inquiry 2: before taking apart the imac, novabench shows 0 score in gpu. is this a concern and implies replacing new gpu? AMD Radeon HD 6970M card shows a number, 109-C29657-10. does it means this gpu was made in 2010 9th week?
by visual observation, no defects at all. is GPU ageing cannot be observed by naked eyes?
inquiry 3: lots of dust were found in 4 ram cards at bottom of imac, where 3 slots were closed with 3 screws. sorry no photos were taken. can those dusts cause frequent imac shutdown? can i instal logic board, not close the screen, and test if the mac works without auto restart?
extremely labour intensive to dismantle the logic board and GPU... taken me one entire day...
( previous efforts include 3 hours call to apple hotline, 3 weekends visits to repair shop, 1 month reading ifixit/this forum/youtuber, and technician guide ...)