What Can Cause Palpitations On The Neck Along With Dizziness?

HELLO, MY FRIEND IS FEMALE ,HYPOTHYROID, ON THYROXINE. FOR PAST FEW MONTHS SHE IS COMPLAINIG OF SUDDEN VISIBLE PALPITATION ON RT SIDE OF NECK. PALPITATION LAST FOR 30 MIN TO 1 HOUR .NO PAIN, FEVER, NOT ASSOCIATED WITH ANY SPECIFIC ACTIVITY. SHE FEELS DIZZY IF THE PALPITATION LAST MORE THAN 15 MIN AND FEELS LIKE HEAVINESS ON CHEST, SHE REMAINS CONSCIOUS ,ORIENTED. SHE HAS BEEN TO PHYSICIANS MANY TIMES BUT BY THAT TIME PALPITATION BECOME NORMAL .SHE HAD HER LABS DONE ANDHAD HER ECG AND HOLTER MONITERING DONE IN NORMAL STATE .ALL CAME OUT NORMAL . PLEASE HELP.