Random screen switch of after SSD update

Hello everyone,

i have a late 2012 iMac 27 inch which was working perfectly fine. For some reason, i thought it is a good idea to replace the HDD with an SSD and ordered the OWC update kit.

I replaced the hard drive without any dramas and at the first start, the display just showed a static signal (like old tv without aerial). I switched it on and off and it worked normally for a few days. Then out of nowhere, the display went black. I powered the iMac off and on and it seemed to work until i tried to reduce the brightness, this caused a black screen again. P+R+Option+Command solved the issues and i switched off the ambient sensor. Now the display goes dark at random times. Sometimes a reset helps sometimes not, at least not straight away so i was thinking that maybe there is a thermal problem but haven’t investigated it any further yet.

Anyone experienced same issues or has some ideas what it could be or what to try?

TIA