I had a moped whose clutch would slip. It sounds and feels exactly like the engine dies. When the moped slowed down, the clutch would begin to grab again and power would resume.
Assuming Sprees are automatic and have a dry clutch, you should take apart your clutch, and sand away the shine from the clutch pads and drum with 100 grit sandpaper. Spray it clean with brake parts cleaner.
The procedure I just outlined, of course, doesn't apply if your Spree does not have a dry clutch.