MacBook Pro freeze

Greetings everyone,

i have a problem with my MacBook Pro 13" with Retina (Mid 2014).

Since nearly 2 month i get random system freezes, most when using safari, chrome or firefox.
The screen goes black or the display content freezes. i can move the mouse, but no mouse click or keyboard press is recognized.

Following things i had already completed:

Disktool: First Aid

Apple Hardware Diagnosis over Internet

Safemode Boot

Install El Captain via USB Stick

Install Maverics via Internet Recovery

Run complete MemTest

None of these points list an error or helping stopping the freezes.

any further suggestions? i'm afraid that there is any hardware problem. unfortunately i'm out auf warranty and had no apple care. is there any way to run a complete system test?