freeze & restart when watching youtube :)

3,1 GHz Quad-Core Intel Core i7 + 16GB ram + NVIDIA GeForce GT 750M 1 GB

problem is funny; when watching youtube videos imac freezes, waits & restarts like nothing happened :) no warning before or after

diagnostics found nothing

• thought it was full of dust and getting hot; opened, cleaned, reassembled - nothing changed
• installed fan control app to see temp and set fan to work based on CPU PECI temp - nothing changed
• I thought HDD was doing this and replaced with SSD - nothing changed
• thought maybe fusion drive was doing this, seperated PCIe ssd with new SSD - nothing changed
• thought maybe it is macos catalina issue and downgrade to high sierra- nothing changed
• made clean macos catalina install and used it as brilliant new without installing expect updates - nothing changed

any ideas??

because of it is happening when youtube videos I thought it is from GPU but after reading some articles saw that it may be because of LCD panel and I was jaw dropping that it was just from LCD panel - is it possible to be from LCD panel?