CO@ Laser lost power aftre 5-10 min

Hello

my CO2 50W laser loses power during cutting.
After about 5 to 10 minutes, the laser begins to lose power.
At first the laser works fine. On the engraved object you can also see the laser doing its job. The laser is connected to a water cooler, which indicates a stable temperature of 20 - 23 degrees Celsius.
After a few minutes, the laser begins to lose power.
I have already checked several things but can not find a cause.
Does anyone know where the problem might be?

Regards Diederik