Cursor stuck in top left corner

Hey guys,

when I start up the mac the cursor is stuck in the top left corner and won't move.

Only clicks with one finger still work.

I found out by accident, that sometimes the trackpad works just fine when the WiFi router is turned off (just a coincidence?) Then, the trackpad works very precisely and all gestures function properly, but only until the mac is put into sleep or is shut down, then it is just like before.

I assume, that this is a software problem, because when it works, it works perfectly.

For now I am using a tethered mouse.

Here is what I tried so far:

• Reset SMC
• Reset PRAM
• Reinstalled el capitan several times, I always erased the SSD completely
• set all other devices (like smartphones or other notebooks) to flight mode, since I was told that sometimes bluetooth signals might interfere in some way with the mac

The warranty period run out a few weeks ago, that's why I'd like to avoid a costly repair.

Thanks a lot in advance :)