I Have Had A Throat Irritation For Many Years Now

I have had a throat irritation for many years now but it is getting worse. I have been checked by specialists after referral by my doctor but they have seen nothing. It comes on at random with phlegm in my throat and feels like I need to scratch it, I think it tends to be worse in winter but can happen anytime. I drink plenty of water & sip cold water slowly when it occurs to ease the soreness, any ideas ??