I think you would benefit from a detox diet. Read the Carol Vorderman book. It could be that you're confusing colds with post-nasal drip/the sort of congestion you get from eating something that disagrees with you, such as dairy. With the diet you eat foods which basically are really healthy and won't cause intolerance, for just 2 weeks. You will feel WORSE initially, but stick with it as all the gunk comes out. After 2 weeks you'll want to keep doing it! Then gradually reintroduce foods one by one and see how they make you feel. I had a permanent cold (runny nose, blocked nose, itchy eyes, asthma etc) all through school. I did a detox diet aged around 30 and for the first time in my life I could breathe through my nose. I haven't gone back to dairy since, not strictly, I just avoid milk, yoghurts etc and have almond milk instead. Everything improved. if Vit C and Echinachea aren't helping, it might just be dairy which is causing your symptoms.