Dog - Scratching

Question
I have a year old border collie/springer spaniel mix named Buster. When I adopted him as a puppy from the Humane Society, he had mange, for which he was being treated.  His hair grew in beautifully, but he has terrible problems with itching.

My vet has said that Buster is allergic to fleas, but we seem to be able to control them fairly well with Revolution.  The vet suggested that he might also have a food allergy.

I read up on food allergies, and I put him on a fish and potato based diet to try to determine what food/foods to which he might be allergic.  I decided/guessed it might be beef, and restricting beef in his diet seemed to help a great deal, at least for a couple of months.  He is eating Beneful's Salmon based dog food and getting primarily chicken or non-beef treats, with the exception of raw hide bones, which are the only things that last long enough to satisfy his need to chew and  keep him from chewing the furniture.  

He was doing so well for two months or so, but now, the symptoms seem to be returning - Scratching/chewing feverishly - behind ears, ribs, backs of legs, base of tail, PRIMARILY AT NIGHT.  If I give him anti-histamines, and distract him long enough for the pills to take affect, he tends to go back to sleep, but I'm not sure if that is actually helping the itching or just making him sleepy.  Topical anti-itch spray seems to help some but isn't very long lasting.

Can you help me figure this out?  I would blame the raw hides, but he was chewing them when the symptoms had subsided.  Is there a significance in when he scratches?  He sleeps on the end of my bed, so I tried washing my sheets and blanket in no-dye, no-fragrance detergent, but that didn't seem to help.

I've tried a couple of fish based foods (without beef), and the Beneful seems to be his favorite.  

Can you suggest something that might give him (and me) some relief.

Answer
If it is a food allergy, you need to leave him on one food for a minimum of 3 months with no other treats or rawhides.  You can also ask your vet to get the dog a shot of cortisone and some cortisone tablets during this food trial to see if there is also an allergy to pollens/molds/insects, etc.  Many dogs have both types of allergies. Flea allergy really responds well to cortisone.