What are these small bumps?

For the past 4-5 months, my dog, Molly, has had multiple pea-sized or less bumps on her back from lower neck to tail area.  They are light yellow-tan in color and seem to come and go. They can be scraped off with a fingernail and fall apart into a gritty material. There is no visible skin inflammation or discharge. Molly frequently rolls over to scratch her back on the carpet, but she has done this since a puppy and I can't say it is any worse. We use Frontline Plus for flea prevention, Oxyfresh Pet Shampoo and we feed her Iams dry dog food supplemented with Pedigree canned food and table scraps. She likes to chew rawhide "bones". Molly sleeps indoors on a regularly washed dog bed, but goes outdoors frequently. We have no other pets, but she is occassionally exposed to my son's indoor cats and the neighborhood dogs.
I took Molly to her regular vet about 6 weeks ago. He did not think the bumps were from flea bites, but he wasn't sure what they  were from. He suggested an allergy from unknown cause and gave her a steroid injection. The bumps improved for about 3 weeks, but are now returning again. Any suggestions?

[Aleda M Cheng, D.V.M., C.V.A]

They sound as if they may be sebaceous cysts which may have an allergic component.  The fact that they resolved with the steroid could also mean that they are allergic in origin.  But, I have seen mast cell and other dermal neoplasms exhibit these behaviors.

Please have your vet perform a biopsy on one or two of the lesion for a definitive diagnosis and for the best appropriate treatment.