I mean 80+ percent CPU and it blocked internet access, when I disabled shields it was ok.
That does sound like a conflict issue...
Especially the part about it blocking off all Internet access for no reason.
Did you try excluding its program files from your other real-time security software and vice versa? Perhaps your other security software was conflicting with Avira, and, afterwards, with Avast.
It is rather impossible that two different security applications can have the same issue in the same config, and in the same system, for the same reason - it is safe to assume that the two applications are not creating the issue solely by themselves.
It is also highly unprobable that this is caused by your system's hardware - it is most likely software-related.