Consider external allergens, such as soaps, fabric softeners, exposure to sun, suntan lotion, foods, medications and vitamins. Consider emotional factors that can contribute to or aggravate the sensation of itching. These include anxiety and even boredom. Often it is difficult to pinpoint the exact cause. Keep a diary of when the itching occurs, the foods you have eaten, activities you've participated in, etc. to provide some clues.
Doctors can do allergy tests to rule out common offenders.
Sometimes everyday stress can manifest itself by itching. If your attacks seem to occur in the summer, ask yourself what is different then that could be causing the problem. The fact that you have no rash or other allergic response could indicate an emotional trigger. Creams and ointments containing hydrocortisone can provide relief in some cases, and sometimes taking a mild tranquilizer during itching attacks can help indirectly. Try to keep yourself busy so you don't think about the itching so much.