Oh poor thing. I'm glad the cause of all this trouble has been found! One thing about space heaters is that they can cause the air to become really dry. Another option would be a hot water bottle punctured at the top and covered in a glove or similar item to avoid burning the chick's skin. A small towel should suffice, too.

Are you sure that the formula you're feeding is at the exact temperature? If it's too cold, it will cause sour crop so you might want to double check the temperature. And don't rule out the thermometer itself! Sometimes, they can be defective, too. I wish you luck! I was never too comfortable with handfeeding myself