-It seems that this is a problem that started with the previous owners, I would look at taking it to the vet to be checked out for any possible internal parasites.

-Since you don't know what the previous owner was feeding, have you tried offering anything other than rats? (rabbits, guinea pigs)

-Are you offering live, pre-killed, or F/T? If you are offering P/K or F/T, how do you present the prey to the snake?