Fibromyalgia: This condition is characterized by widespread musculoskeletal pain accompanied by fatigue, sleep, memory, and mood issues. The normal brain MRI and normal examination, along with the nature of the pain (intermittent limb pains), make fibromyalgia a plausible diagnosis given its prevalence and the symptoms described.

Complex Regional Pain Syndrome (CRPS): Although less common, CRPS could explain intermittent limb pains, especially if the pain is confined to one limb and is accompanied by other symptoms like changes in skin temperature or color. However, the absence of specific triggers or a history of injury might make this less likely.
Peripheral Neuropathy: This could be a consideration if the limb pains are associated with other symptoms like numbness, tingling, or weakness, though the normal examination might argue against this unless the neuropathy is very mild or intermittent.
Vasculitis or Vasospastic Disorders: Conditions like Raynaud's phenomenon could cause intermittent limb pains due to vasospasm. However, these would typically be associated with other symptoms like color changes in the fingers or toes.

Multiple Sclerosis (MS): Although the brain MRI is normal, MS can sometimes present with normal imaging early in the disease course. The intermittent nature of the limb pains could be consistent with a relapsing-remitting pattern, and MS is a diagnosis that could have significant implications for treatment and prognosis.
Lupus or Other Connective Tissue Diseases: These can present with a wide range of symptoms including musculoskeletal pains. A normal examination and MRI do not rule out these conditions, as they can be serologically active without overt clinical manifestations at the time of evaluation.
Osteoid Osteoma: A benign bone tumor that can cause intermittent pain, typically worse at night and relieved by NSAIDs. This would be an important diagnosis not to miss due to its treatable nature.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Some types can lead to musculoskeletal pains, though this would typically be accompanied by other signs like hypermobile joints or skin hyperextensibility.
Mitochondrial Myopathies: A group of disorders caused by mutations in the mitochondria, which can lead to muscle pain among other symptoms. These are rare and would likely be associated with other systemic symptoms or a suggestive family history.
Sarcoidosis: Can cause musculoskeletal symptoms including limb pains, though this would typically be part of a broader constellation of symptoms including respiratory or dermatological manifestations.