Product review (5 stars): This was the culprit
LG oven has been acting badly for some time now. Flashing the infamous F-3 error and shutting off, taking considerably longer to cook things, etc. The internet suggested that the thermistor was defective. Ordered the replacement from Amazon, and 10 minutes after it arrived the oven was back to brand new. Shame products today don't last as long as they used to, but at least this was an easy fix and relatively inexpensive.