Update:
To date, I have been using a hotplate where the element is mounted under a cast iron disk. Last night, I procured a hotplate with a bare, spiral element and gave it a try. The colloidal gold turned out perfectly. I'm wondering if the cast iron was releasing something into the air. I know I'm grabbing at straws here, but there has to be something that changes periodically. As I said...I keep using the same materials and amounts and the successes and failures keep alternating. Doesn't make sense.
On the upside, I just about have enough money for a hotplate stirrer. I have a feeling that once I obtain this, I will start seeing more consistency.