Hey,
I read your post and i just wanted to let you know, I have been suffering from the exact (and i mean exact) same kinds of headaches you are describing for almost two years now. Out of nowhere, I'll gradually get this headache centered around my right eye (eyebrow especially) that doesn't make me nauseous or light headed or anything, it just hurts. I'm a huge hypochondriac, and I would read about brain tumors and scare myself so bad that I would have panic attacks.
Eventually I couldn't take it anymore and I got a CAT scan done, which found nothing. I saw about 4 different doctors who told me not to worry; what made me feel better, and what should make you feel a little better, is that there are a lot of bad symptoms other than headaches that are tell-tale of brain tumors (and i'm guessing you're not having anything else strange other than headaches) such as vomiting, dizziness, loss of sensation in limbs, etc.
To make yourself feel better, I would say definitely go to a doctor, you have every right to be concerned about your own body. But I just wanted to tell you I have been having that same kind of headache (comes and goes like you said) for almost two years now and I'm fine. Excedrin Migraine has helped me tremendously when I get them.
Anyway, good luck, and don't be too scared -- you're not alone with those headaches.