Hello, me again!
Eventho there is no response here so far I will give some update.
I bought affinity designer eventually and it was running stable for a few days now. I feel that the mentioned problem occurs when I use vector brushes and paint with them.
It does not seem to make any difference whether I use pressure control or not, neither does the speed I am drawing at seem to make any difference.
The break down appears after a few moments to a few minutes after I start using the brush.
I was not using any brush for a few days and I had no problems at all.
This brush pack seems to make most of the problems, but the same happens with default brushes after I reset the program. (https://gumroad.com/l/frankenmarkers)
I hope this helps.
best regards!