Hi again,Customer and thanks for your additional information.
It's possible that the new brush is a little too rough on her skin and is causing the white flakes you're seeing; it's also possible that these are flea eggs, and even if she's indoor-only, she can still get fleas, brought in on humans' pantlegs and shoes. This happened to my cat, and imagine my surprise, as we live 3 stories up!
If you see no black specks or other evidence of fleas, then the white flakes are most likely dandruff, as you suspect. I'd stop using the Furminator at this time, and bathe her in a cat shampoo to alleviate dry skin, from Drs. Foster and Smith:
Also, use some of the other products to moisturize her skin and keep her fur in good condition. You can find many of these products in major pet supply stores, or order online, of course.
If it's colder where you live now, her fur may be naturally drier, due to the season, so you can run a humidifier in your home, or in the room she spends the most time in, to add moisture to the air and back into her coat. Also, make sure she's drinking sufficiently.
It's not necessary to bathe her, unless she gets into something sticky, oily or toxic; frequent bathing dries out the skin, but I wouldn't call every four months, 'frequent'.
I hope you see an improvement, soon. If not, an evaluation by your vet would be a good idea.