Interestingly enough - I am having the same problem with Big Sur. But I also had it on High Sierra since December or so. It only seems to happen when I have external monitors connected. On High Sierra, changing from HDMI connection to Display Port connection solved the problem. But I upgraded to Big Sur the other day and now it is happening with both. Model Name: iMac  Model Identifier: iMac19,1  Processor Name: 8-Core Intel Core i9  Processor Speed: 3.6 GHz  Number of Processors: 1  Total Number of Cores: 8  L2 Cache (per Core): 256 KB  L3 Cache: 16 MB  Hyper-Threading Technology: Enabled  Memory: 64 GB  System Firmware Version: 1554.80.3.0.0  SMC Version (system): 2.46f13