Shirley O. Corriher, in her recently published book Bakewise, says that the oven temperature can drop 150° or more if the oven door is left open just thirty seconds! The oven can then take several minutes to come back up to full temperature.
So if your dish is almost done cooking and you're checking it for doneness every few minutes, this means that the oven never has a chance to recover. No wonder it sometimes seems like a dish is taking forever to finish cooking, right?
Sigh... Live and learn...
P.S. We'll be doing a full review of Shirley O. Corriher's book soon! You can also order your own copy for $26.40 from Amazon.