Hardware New 3DS XL acting weird, Freezing

[Oliver-Hoschi]

Hello together,

my new3ds xl is making some strange problems.
i modded it with cfw years ago and everything worked fine, until a few weeks ago.
The home menu works fine, when i start a game it let me play it for about 5 to 15 minutes, after that the FPS drop to 1 and the sound also.
If i´m fast enough i can press home and close the game, home menu works in this moment at normal speed.
If i´m not fast enough, the 3ds freezes completely. If it freezes completely and then shut down by myself, it wont turn on for about 30 minutes or so (Blue light but no boot).
For me it sounds like a temperature problem, but on the 3ds there is no fan or thermal paste to change, so one of the chips must be faulty right?

Things that i tried so far:

Updating the CFW (no change)
Replace the SD Card 2 times (no change)

So what could i try next? Maybe reflow the ram or the gpu? i have a SMD rework Station with Hot Air and so.
But i don´t find tutorials or videos which chip to reflow.

Any ideas?

thank you in advance and please excuse my bad english, its not my native lang.