Imac starts typing by itself after sleep

This is the issue.....

My iMac keeps waking from sleep since 3 days now. It took some time to figure out why, but now I found out that it types random 5's after waking up. I tested this by opening a text document and then put my iMac to sleep by clicking the apple icon and then choose sleep. I hear the computer shutting down but almost immediately after that it wakes up again and the 555's appears in the text document. After a while it stops repeating the 5's like now when I am typing this, it does not happen.

As soon as I switch my iMac to sleep again, it will start over.

I have recently replaced the harddrive to SSD which went smoothly and worked for 3 weeks without a problem, so I would not think that is the problem here. There have also been changes in55 router and NAS a few weeks ago but that should also not have something to do with it.

The keyboard I am using is wired and the 5 button that causes the problem is the one from the numeric pad. I found that out by popping up the virtual keyboard to see which button was causing the problem.

You would think it is the key that is sticky, but why does it have a connection with the sleep mode of my iMac then?

iMac 27" mid 2010 on High Sierra

I hope there is someone who can help me out with this problem. 55555 5oops and there it starts again 555