robtillaart:
I suspect the problem lies in interrupt handlers,
An interrupt at the wrong moment can indeed "ruin" measurements, but is your code receiveing bytes while measuring? If no then there are no interrupts...
Nothing else is happening when I am trying to take these measurements, but there is an open serial port (using the Serial commands).
For all intents and purposes, this appears to be resolved on my part by moving to an older Arduino IDE. I do apologize for being a "this isn't my problem" type of person, but I simply don't have time to troubleshoot this. All I can say is I've found what appears to be a regression bug, and I want to make sure the right people know.