Seborrheic Dermatitis: This condition is characterized by a dry, itchy rash often found on oily areas of the body, such as the face, including the area between the eyebrows, and can also affect the back. Its intermittent nature and the described locations make it a prime candidate for the single most likely diagnosis.

Atopic Dermatitis (Eczema): Although more commonly associated with flexural areas, atopic dermatitis can present with itchy, dry rashes on various parts of the body, including the face and back, especially in individuals with a history of allergies or asthma.
Contact Dermatitis: This could be either irritant or allergic in nature, resulting from exposure to substances that irritate the skin or trigger an allergic reaction, leading to dry, itchy rashes.
Psoriasis: While typically associated with well-demarcated, scaly plaques, psoriasis can sometimes present with more subtle, itchy, dry patches, especially in its early stages or in certain locations like the face and back.

Cutaneous T-Cell Lymphoma (Mycosis Fungoides): Although rare, this condition can present with skin patches, plaques, or rashes that are itchy and can mimic more benign dermatoses. It's crucial to consider this diagnosis to avoid delayed treatment.
Sarcoidosis: This systemic disease can cause skin lesions, including rashes, that might be itchy and appear in various locations, including the face and back. Missing this diagnosis could lead to untreated systemic disease.

Pityriasis Rubra Pilaris: A rare skin disorder characterized by constant itching, palmoplantar keratoderma, and reddish-orange scaling patches, which could initially present with dry, itchy rashes.
Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness. The rash can be itchy and appear on the face, including the area between the eyebrows, and on the back, among other areas.