knitr in Rstudio processes file, starts pdf display, then locks up with blank windows

I'm working on a paper (Rmd file for pdf output -- R-Journal eventual destination) which is relatively complex.
Every few cycles of minor edit, render, knitr in Rstudio goes through the processing, the pdf display window
comes up momentarily with material (too quick to see if it is updated), then goes blank, as does the Rstudio
IDE. Can close the pdf view window, but X on Rstudio does nothing. Taskbar "close" fails. I use a terminal
and
ps ax | grep studio
kill (1st reference process)
to recover and then restart Rstudio. I was rebooting, but that was really tedious.

RStudio Version 1.2.5033
R version 4.0.3 (2020-10-10)
Linux john-J19B 5.4.0-54-generic #60-Ubuntu SMP Fri Nov 6 10:37:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
(Linux mint 20)

Suggestions of diagnosing welcome.

JN