Too hard to say w/ out the basics. Lots of things can cause this... Start at the bottom of the pile, and get a comp> test done. Any "Mechanic/Shop"
can help you out on this for a few quid. If I had a scoot that did as you describe... That's where I would start.
Ps... Don't ask them to fix it, Just ask for a compression test. 5-20.00 bucks if not free.