TUMBLEWEED Tumbleweed overheats in KDE but not in XFCE

since a few months I encounter the following:
After starting up my desktop with Tumbleweed-KDE I use it on and off during the day. After about 6 hours, suddenly the temperature of the PCI adapter rises and the system is almost unresponsive. (almost: about once every 3 minutes a new value becomes visible in sensors, all the time well above acceptable values)
The command in the console “watch sensors” then shows a temperature of:
composite: 72 C
Sensor 1: 60 C
Sensor 2: 90 C.
Testing promptly after startup with 10 x writing and deleting a video (4 GB) raises the temperature to about 85 C, the system will not hang.

Since looking through Internet I found that KDE is resource-heavy, so I tried a lighter GUI: XFCE.
The test with 10 x writing and deleting gave the same temperature effect.
But even while using the computer on-and-off during 24 hours, no hang occurred.

I would really like to know what happens in KDE that does not happen in XFCE, if only a pointer to a package that causes sudden overheating after about 6 hours.

Daniel