Unless there was something way off in the stop pH or something, I'd rule out acid stop/pinholes, especially since the developer was D76 (no carbonate). I have a hard time believing this rare phenomenon would occur with a venerable product like Plus-X.

I also highly doubt any of this is related to temperatures during processing - unless you had a serious temperature shock or very high processing/drying temperatures. Again, Plus-X is not a film that would be sensitive in this particular way even to fairly sizeable temperature fluctuations during processing (image quality might be compromised, but the film should/would not be damaged in the way OP presents).