Holy smokes... I just did some more digging and I found the culprit. I looked through all the "// TODO" comments in my code and then I remembered I had an odd issue with a signal/slot connection on a QTimer* that produced a warning in the debug output. The timer just wouldn't fire. I commented out the connect() command because I had more important things to do. When I came back to this snippet of code, I noticed that I hadn't initialized the QTimer*... I feel like crap for wasting your and my own time. Too bad Valgrind didn't detect this, because then the error would have been exposed much earlier.

Thanks for your help, though!