What Causes Body Tremors And Racing Heart Beats In An Elderly?

Hello, It s my mother she is 92 years old she has had this condition for at least 15 yrs. She only shakes at night. She shakes like she is cold, her hands shake, she looks so nervous We have tested her blood count its a normal count.She only has HBP and it is always in control. She weighs 130 is 5 2. She takes lorazoapan to calm her down and to sleep. very low dosage. She eats well. Drinks 1 cup of coffee in the am.. does not drink or smoke. Some time she says her heart races. Any help would really be appreciated... Roz