Ch7/pvlv reliably freezes for many users

[StephenJRead]

three or four of my students reported that PVLV completely freezes when they are trying to do any of the questions past 7.7. Some of them were not able to even do 7.7. This occurs for both Mac and Windows users. I had the same problem running Mac Catalina. Note this is not a crash, but instead is a hard freeze. Nothing on the interface is responsive. No messages in Terminal.
But not all students report this, many were able to finish.