their are lots of different reasons for hr racing both up and down and i agree with jerry if u give more info we may be able to give a better answer ,but in a healthy setting ,too much caffeine ,stress,dehydratation ,thyroid ,exercise etc .if u feel your heart is kicking up for no reason a holter monitor may show up what is going on during these periods
Do you have any numbers for your "racing" HR?
Working out will cause the HR to go up/down depending on your workout intensity, I"m sure you know. But your question isn't clear on the conditions that cause you concern.