Try using a penis health creme. These are made for rashes, dots and things of that nature that occur down below. Get one with all natural ingredients though. Good luck.
Hello,
I cannot confirm anything without examination but it can be due to allergies, hives which come and go, friction rub or chafing during intercourse or some form of dermatitis. I am not aware of any STD which presents with such rashes that come and go.
Take some Benadryl when it appears the next time and apply some calamine lotion. If the symptoms persist then topical and orals steroids may be needed. Please consult a dermatologist in that case.
I hope it helps. Take care and regards.