Nintendo switch crashes apparently randomly

Hi everyone, just wanted to ask for some help with my recently purchased, second hand, switch.

As far as I can tell,according to the previous owner, the switch was working properly until the charge USB broke…after that, he decided to fix the console in a general reparation store to avoid waiting too long for a Nintendo fix.
After repair, the charge of the port was working again (the whole connector board was replaced) but, the console started to show a different issue, it will hang/crash randomly and the console wont work through the dock (no image when using it). I decided to pull the trigger as the console was at least working, and bought it. As the seller told me, it works, and also crashes apparently randomly, I mean, I can go al the way playing with it for maybe 1h long with no problems, but other times it will crash after 15 minutes…thing is, it will end crashing sooner or later.
I opened it and changed the thermal compound (both, the one under the tegra chip AND, the one over the copper pipe touching the metal backplate behind the console…it still crashes!. I dont feel like the switch it getting too hot at all, in fact the fan dont usually start spinning too often, it works though when needed but even then not too hard and the console only feels slightly warm at most, so hardy a heating issue imho.
The “kind” of crash Im getting is: The screens freezes and typical sound loop. The only way to stop it is by pushing and hold the power button for 10 secs.

The question is, what do you think could be the issue?, do you think I will be able to repair it myself?..I have certain weldind abilities and material for making a new soldering where needed if that would fix the issue but Im not sure what the problem could be at the moment (at first I though it could be a heating problem but now Im not sure).

thanks a ton!