Now & then on backs of arms I get lots of tiny bumps almost like a rash with no redness or itching they hang around for few days comes & goes causes?

Possibilities. Could be keratosis pilaris. Are they firm bumps? If so it can be caused by keratin clogging hair follicles. Gently using a "buff-puff" or other time of mildly exfoliating cloth while in the bath or shower can help unplug the follicles; then pat your self dry and allow to air dry prior to using a mild retinoid; like retin a (tretinoin) which is a prescription or an over the counter retinol product.
Dry Skin. There is something known as keratosis pilaris, flesh colored, little pimples or goose bump-like, non-itchy, likes to camp out on triceps or sides of arms, outer thighs and occasionally cheeks on the face. Is usually due to dry skin. No scrubbing! use something like amlactin or cerave sa lotion twice daily.
Keratosis Pilaris. Keratosis pilaris is harmless (benign). It seems to run in families. It is more common in people who have very dry skin, or who have atopic dermatitis (eczema). The condition is generally worse in winter and often clears in the summer. Treatment can be with otc cerave cream sa.