Without a close physical examination of the spots, it will be hard to provide much insight into your condition. If you are getting a lot of different conflicting opinions from your different doctors
, you may want to see a dermatologist
(if you have not done so already), as they may have some insights that your primary care doctors
The fact that the spots come and go is a bit unusual, because most of the standard causes of brown spots on the forehead should not do that. For example, conditions like melasma and sun spots should not disappear and, in fact, tend to get worse over time.
One condition that does wax and wane, and could potentially be worse with sun exposure, is tinea versicolor, which is a fungal infection of the skin that is also known as 'sun fungus' particularly because of its tendency to worsen in the sun.
I suggest that you start by seeing the dermatologist. If they do not have any immediate ideas, they may recommend a period of observation or, in certain circumstances, they may recommend a biopsy just so that you can get to the bottom of things and stop wondering what this is.