Major service for Trek 4300

I took my MTB in for a Major Service in March 2015 at Lynnwood Bridge, I paid over R1k for this service. I bought the Trek 4300 at this shop a few years back and serviced the bike regularly. I received my bike back - went to cycle for fun at a track - and found that my gears stated changing itself. I took my bike back on 21 May - where the assistant understood and said they will investigate this matter and I received it back on 22 May. The same assistant ensured me the bike is ready and all the parts was checked as per original service paid for. I did a MTB race on 31 May, only to find that my lowest gear sounds now like a Harley when I peddle. This is very poor service and disappointing because I only received good service from them the past few years.