Suggest Remedy For Frequent Dizzy Spells

hello!I am a 30 years old man.I have been suffering headaches and dizziness for seven years but in summers i feel well. nowadays no headache but still sometimes dizziness esp.when the back part of my head is in action.when it's cold my nervous at the bach of the head start vibrating than comes dizziness.i play football quit normal and I practise some boxing I don't feel nothing only when I stop I can't stand I have to lay down .I consulted doctors the answer was everything was ok I mean no heart ,head, diabet problems.would you please tell me what could be the problem.thanks