Some flashing from Cichlids can be normal. As I understand it, there are a few primary culprits when it is excessive.
Assuming that your aquarium is cycled and you have 0 Ammonia and Nitrite, make sure that your Nitrate level is 10ppm or less. Cichlids are sensitive to Nitrate and higher amounts can cause flashing.
The other primary consideration would be some type of parasitic infestation. Things like ich and gill flukes will often cause flashing. These parasites can (or do) set up shop on the gills of your fishes where it can be very difficult to detect them.
Another possibility is that you are using a low quality water conditioner during water changes and trace elements of chlorine or chloramine are not being neutralized.
These are the first three areas I would look for a solution to excessive flashing absent any other indications of illness.