questions concerning high pulse rate.

Hello .I am a 61 yr old female who has been having problems with high pulse rate.  It ranges from the 120's to high 160's.  The Dr. put me on profenone 150 mg. I take it three times a day  but it hasn't helped.  The Dr. then told me to double them 3 times a day.  My pulse rate still goes from the high 120 to low 150.  I have passed out twice while driving a car, totaling one and causing considerable damage to the other. Needless to say I am not driving anymore!  My ques. is can the high pulse rate cause someone to pass out. Or can the medicine cause it?  If anybody has any answers to this question , please let me know. Also how long should you let this go on , having a flunctuating fast pulse?
                       Thank you very much,   nana6pg