Gastroparesis (slow stomach emptying) is a common feature of Parkinson's Disease. Levodopa (Sinemet), a common medication for Parkinson's Disease, can make gastroparesis worse. Gastroparesis effects how the levodopa is absorbed and used by the body. This study will explore the possibility of using Erythromycin, a drug commonly used (off label) for gastroparesis, along with levodopa to determine if there is improved levodopa absorption and motor function.