Heart problem or anything else??

For last 1.5 yrs i experienced heart palpitations for 6-7 times.. During palpitation the heart beating was so faster than usual.. Heart palpitations happened only at evening or night time. (Not at the morning time) Whenever happened this i become very fear, nervous and panic.. and feel like that i am going to unconscious and die. As of now, i visited more than 10 doctors including 4 cardiologist.. In last 1.5 yrs I did 4 ecg 2 echo test 2 tmt 2 chest xray Thyroid Sugar test All above tests were normal time and again.. Only LDL and BP are little high, i taking medicine for that. Note i am non smoker and non alcoholic and don't have family history.. Despite all the tests were normal what maybe the cause for heart palpitations. There are no significant/noticable symptoms i feel during heart palpitation.. Please help doctors...?