Are you using the antibiotic cream for every ride, or only since the sores developed? Is it a petroleum jelly-based preparation? Are you using it when you're not riding?

I haven't had to deal with this problem (thank the Goddess), but if acne prepartions are helpful, it sounds like something that needs to dry out and get some oxygen. Benzoyl peroxide is in acne goo b/c it delivers oxygen into an anerobic sore, killing off the anerobic bacteria.

Just found this: "Acne is formed when skin cells inside hair follicles shed too fast and clump together, plugging up the follicle and causing a pimple. Salicylic acid helps slow down shedding of the cells inside the follicles, preventing clogging. Salicylic acid also helps break down blackheads and whiteheads."

Seems if you're using a petroleum-based product it would keep the good oxygen from getting in there and killing off the buggies.