iMac freezes + WiFi card issue

Hi all and thanks for stopping by.

My old buddy iMac is having some issues:

1) it freezes. most of the time when you try to run a game or Geekbench GPU test. it's just 100% that it will freeze within 20-90 seconds after you start a game / Geekbench test. Rarely it can freeze during ordinary web surfing or even while loading the MacOS (the Apple logo and progress bar at ±50%).

No glitches or other GPU artefacts. Just a frozen image and cursor is not moving (sometimes you still can move it, but nothing works).

2) it shows "WiFi hardware is not installed", though i have my bluetooth working. Heard that there was an issue with late 2013 A1419 model when malfunctioning WiFI card can lead to freezes so i tried to replace the WiFI card but had the same problem - "no wifi hardware installed" + freezes.

I even tried to run the iMac without WiFi card, but freezes are still there.

I also did MacOS reinstallation, SSD check (all good), Geekbench CPU tests (all good).

I'm thinking of GPU issues since i can force these freezes by just running a game / Geekbench GPU test, but the problem with WiFi card ruins all the theory of malfunctioning GPU.

any ideas?

Spec:

iMac late 2013

CPU: Core i5 3,4GHz

Ram: 8GB DDR3

1TB SSD

GTX 780M 4gb

MacOS Mojave 10.14.6