Diagnosis-help. Why is it repeating?

I have been facing a condition wherein my pulse rate shoots up (i habe been diagnoised by tachycardia), along with severe sweating, i can listen to my heart beat in my ears. I becaome very weak & everything starts to look yellow. Then i almost faint (though im conscious n my eyes r open). This remains for few minutes. In the mean time if i get water n a place to sit, i become well in lesser time. All tests like EEG ECG thyroid, Doppler test, all blood tests have been done but nothing came out. This is repeating n im now scared of standing for long (as it happens mostly when im standing, during mornings). Im not sure which doc shud i consult, a cardiologist or a neurologist? Cardiologists say that i hv tachycardia n thats nothing to get worried about? Then why is it repeating?