I have sore hard lumps around the crown and back of my head. they eventually scab over and sometimes go away while others appear. what causes this?

Several options: Without examining you i cannot give you a 100% answer. However, some thoughts are acne keloidalis (sometimes seen with really short haircuts), could also be a form of scalp cellulitis. I would suggest you see a dermatologist who can help clarify the answer and best recommend treatment.

R/o psoriasis/eczema: Get checked with your primary doctor to get ruled ot of the above- seborrhic dermatitis-a type of eczema , psoriasis. Also get ruled of other things like fungal infection of scalp/small pustules from staph, sterptococcus gp .Of organisms(bacteria).