When running the test windows is not running , thats probebly why it didn't shut off or crash during the test. And if memtest finds thousends of errors then im pretty sure it is the ram that is the problem.

Any change of borrowing a compatible stick to test with?

Nope , you get what you pay for , that counts for almost everything but specially for ram i think.

Is it impaortant that I stick with memory that is on the motherboard qualified vendor list?

Nope. As long as the specs are compatible it doesn't matter.