Why is it happening? MBP struggle to power up, no cold joints or crack

Hi guys i need a bit of advice about what's going on here.

i have a macbook pro 17 mid 2009 which struggle to power up, let me explain better :

when the first time i press the button , usually it briefly lights up the sleep led and shuts, then you press it again it reaches the apple logo and shuts again.

if you repeat pressing the button , and after few shuts down , the mbp manage to reach the desktop working fine for days.

I had one similar problem with a late 2008 15" and problem was a cold joint.

But on this machine i've checked power rails and buck regulator and caps (like c7771) which usually are problematic.

Everything shows up at the correct voltage.

Cleaned the motherboard twice, did check for any cold joints.

Motherboard on its own (no external components) stays on without shutting down.

I tried to trace the components that maybe are drawing too much current which could lead to the problem.

Battery has 28 cycles, it doesn't seems to have a faulty cell.

I'm lost

hey mayer , thanks for the answer!

Laptop has been in storage for a while , then reutilised .

battery is strangely at 90% of its original capacity after 28 cycles, I'm trying to recalibrate it at the moment.

screenshot attached!

regards