Failure to boot w/ GPU installed: fans+lights - no beeps or display

[egoistcat]

i believe this is everything important i've done to the pc

crash under load (first time ever)
restart
crash under load
failed to restart - fans spin + lights blink but no display or beeps
restart 20min later (assumed overheat)
crash under load
failed to restart - fans spin + lights blink but no display or beeps
failed to restart after overnight
new mobo, cpu + ram installed
launches into bios fine
(various problems failing to get win7 to work)
format hdd and install win10
system works fine w/ gpu
multiple restarts due to driver installations incl. gpu drivers - no issues
crash under load
restart
crash under no load (just discord call + 2 chrome tabs)
failed to restart - fans spin + lights blink but no display or beeps
remove cmos battery to reset bios
failed to restart - fans spin + lights blink but no display or beeps
replace gpu with older gpu (no psu pins only mobo power)
launches fine
put everything in pc under load (only 50W psu draw) - no issues

my current specs are

gigabyte b360m d3h mobo
i5 8400 2.8ghz hex core coffee lake
2x4 2666mhz ballistix sport LT ddr4
amd radeon r9 280 3gb GPU (the backup one im using now is a radeon hd 6670)
700W psu : rs-700-acab-d3

first glance it looks like the gpu is failing somehow, but having just forked out for the other replacements i'm hesitant buy more parts without knowing for sure. have checked and rechecked all cables, reseated all components, dusted components throughout. usually i'd keep working at it but after 3 days of scrolling through boards like this one for various issues i'm ready to give up

also worth noting that if i fully unplug the gpu i can launch on dedicated graphics no problem

any help/advice appreciated

UPDATE: bought new PSU as that was next on the list to replace anyway - issue persists. considering the GPU is now the only old thing still in the machine i guess it can't be anything else. a bit bummed out it looks like i have to buy a new gpu as well, is there anything else it could potentially be?

 

[Greeko85]

Well if the issue occurs when the system is under load it could very well be your psu playing up. How old is the unit? Is the system plugged straight to the wall or a power adaptor/strip thingy?

 

[egoistcat]

-snip-

psu is about 2 years old, there's also a possibility it's a refurb but for the life of me i cant remember

it's plugged into a strip but my previous rig (older cpu/mobo/ram) worked without problems for a few months in this setup. the first failure happened the day i was ordering new components anyway. cheapy power strip though so no surge protection

 

[Greeko85]

I'd give it a try plugging it straight to the wall just to eliminate that being a cause (however slim). If it continues any chance of borrowing a psu from a mate?

 

[egoistcat]

tried but no dice

have asked around + might be able to pick one up tomorrow, but my gpu alone wants 500W and most of my mates have pretty standard rigs so unsure if i'll find one big enough