Graphics Issue

Recently installed Big Sur on my 2011 iMac. Everything seems fine but sometimes the graphics start to glitch. Tried to fix it with the screen saver method and reinstalled the root patches but it doesn’t seem to fix the problem. Any advice or tips how to fix it?

Processor: 3,06 Ghz Dual-Core Intel Core i5 RAM: 16GB 1333 Mhz DDR3 Graphics: Radeon HD 4670 256MB