I'm building an open source, commercially available eurorack module which is based on a stm32 chip. The problem I'm facing is that when the module boots up sometimes it is working as it should but most of the times, it seems to be working at a lower rate and it has unpredictable output.
I checked all the voltages from the regulators and I checked the soldering of all the parts and everything was right. After checking the 8Mz crystal I found that the one end oscillates as it should but the other does not. When the module works as intended, it oscillates nicely but after a while it goes berserk along with the module outputs.
Now, I double checked the values of the capacitors (20 pF) and the value of the resistors going to the stm pins (100 ohms). I replaced both the stm and the crystal with new ones. I reflowed with hot air the crystal and the stm but the behaviour is the same.
What strikes me as quite peculiar is the fact that sometimes the module works but after some seconds, of even minutes (after a reflow for example) the crystal stops oscillating as it should. Can anyone tell me what can cause such a behaviour?
The schematics are here: pdf link
Edit: Schematic of crystal oscillator section