Gear change handle bar.

Lambretta had new gear and throttle cables a couple of weeks ago. Everything lined up nicely and gear changing was nice and tight. Done about 30 miles and the gear change became a bit sloppy. Back it went to the garage and they re adjusted the cables. Done about 50-100 miles and the gearing has become sloppy again. This is the reason I had the previous cables changed as I thought they were at the end of their so called use by date and it would eliminate this problem I have. The garage has changed parts inside the handle bar which could cause this sloppiness problem. Also while at traffic lights this morning about 20 miles into a ride the scooter was moving forward when the gear lever was held in and under a little bit of throttle. The model in question is a series 2 tv175. Any help or suggestions would be appreciated.