No real idea, but here is what I personally have horrible itching from: taking Percocet pain meds, some penicillin scrips, cheap soap (especially the antibac stuff like Dial and deodorant awfulness like Irish Spring), mineral oil in body lotions, the chemicals in orange sodas like Squirt and orange Crush, too much canola oil in the diet, Downy fabric softener, green-tinted OTC liquid and liquigel cold and flu remedies, eating at McDonalds too often (now a permanent part of my past), and processed box foods with artificial colors and flavors and preservatives.
My hubby once had an allergic reaction to a sunburn, and the itching was so intense that I sent him to the ER in the middle of the night. He came back with heavy duty antihistamines and thanked me - he was half-crazed, it got so bad.
I second nameless's suggestion to go to a good dermatologist. It really could be anything from chemicals in your environment (even at work), to foods, to a fungal infection, to mites (seriously), to medicines. At least you can rule out a lot of things, if you go to a skin specialist, as your GP hasn't helped. Good luck - I hate itching.
I have a mantra that I have spouted for years... "If I eat right, I feel right. If I feel right, I exercise right. If I exercise right, I think right. If I think right, I eat right..." Phil-SC