if it appears in 2 different browsers (with different rendering engines) then it is unlikely to be browser related, if it was happening to lots of people then it would have received a lot complaints.
my advice would be to run hardware diagnostics ( full stress test ) on both the PC and the graphics card ( if it is a discrete card )
artefacts like that can be caused by failing memory in the graphics subsystem or by overheating of hardware components.
if all the tests pass then at least you know your pc is healthy, if any fail then there is a hint at what might be going wrong.
most manufacturers have diagnostics available to download from their websites