Looks like there is already an issue for this:
It looks like this might be an issue with how the browser frees up memory. I suggested that perhaps Cesium can allow users to re-use WebGL contexts. In the mean time you might try to dig into the source and do this yourself (and if you get it to work we'd absolutely appreciate a contribution!)
Or perhaps you can destroy all entities and hide/re-use the viewer as a workaround?