 The purpose of the study is to assess the efficacy and safety of a range of doses of SCH 420814 preladenant when used together with a stable dose of L-dopadopa decarboxylase inhibitor to treat Parkinsons disease. In this study, we will be comparing 3 doses 1 mg, 2 mg, and 5 mg taken twice a day of preladenant with placebo sugar pill. Following an Interim Analysis temporary hold for new enrollment-ongoing subjects will continue on treatment to review drug safety, a new dose group of 10 mg taken twice a day may be added. 