Thanks for the update britzkopf.
Regarding ram:
You can bring up the option to do a fair ram test simply by pressing del key a few times when you boot.
Back in the old days, ram test was mandatory. Then it was optional but default was to do it. Now it's optional and default is not to do it. There are two reasons why it's not done by default anymore:
1. ram is more reliable.
2. it takes a lot longer than it used to.
One could argue whether it's more due to reason 1 or reason 2 - still: it's a critical component and does fail sometimes. It can cause all the symptoms you are seeing, granted it could also be due to something else...
~R.