I have a janome and have not had his problem with it. It runs very well and is quiet. Ring the dealer you purchased it from and see hat they say.
P.s. have you checked the lead? Is it coming out of he holey thing on machine. Any holes etc especially if you have young animals. Are the leads inside the plug loose.? Al, these have caused similiar problems with other machines. Slows down then comes back when wire is jiggled.