Cycles viewport flickering when rendering 3D models typically stems from low sample counts, real-time path tracing limits, overlapping geometry, or outdated GPU drivers.
Cycles uses path tracing, which needs multiple samples to smooth noise—too few samples cause random pixel variations that flicker as the viewport updates. Duplicate or overlapping faces in your model can also create glitches. Outdated GPU drivers may not support real-time rendering well.
To resolve it, increase viewport samples in Cycles settings, check your model for duplicate geometry, or update your graphics drivers.