Sudden itchy red raised bumps all over face. they disappear after a few hours or so. this occurs once in a blue moon but definitely not a 1 time thing?

Allergy: I sounds like you may be allergic to something in your environment. The fact that it occurs infrequently could mean it is something which is seasonal. See if there is a consistent time of the year or weather condition that is associated with the rash or any environmental condition. At the time you have the rash it should be examined and if necessary a biopsy should be done.