Temporomandibular Joint (TMJ) Disorder: This is the most likely diagnosis given the symptoms of jaw pain and stiffness, especially when chewing or eating. TMJ disorders are common and can cause intermittent pain and stiffness in the jaw.

Bruxism: Grinding or clenching of the teeth can lead to jaw pain and stiffness, especially if it occurs at night. This condition is often associated with stress and can be exacerbated by chewing or eating.
Dental Problems: Issues such as cavities, abscesses, or gum disease can cause pain in the jaw, especially if the problem is located in the upper or lower molars.
Sinusitis: Inflammation of the sinuses can cause referred pain to the jaw, especially if the maxillary sinuses are involved.

Giant Cell Arteritis: Although rare, this condition can cause jaw claudication (pain when chewing) and is a medical emergency if not treated promptly. It typically occurs in older adults and can lead to blindness if not addressed.
Osteonecrosis of the Jaw: This condition, often associated with bisphosphonate use, can cause severe jaw pain and is important to recognize due to its potential for significant morbidity.
Trigeminal Neuralgia: A condition characterized by sudden, severe, shock-like pain in parts of the face, which can be triggered by chewing or eating. It's crucial to diagnose and treat to alleviate pain and prevent complications.

Sarcoma or Other Jaw Tumors: Although extremely rare, tumors of the jaw can cause pain and stiffness. Early detection is crucial for effective treatment.
Eagle Syndrome: A condition where the styloid process is elongated, causing pain in the jaw and face. It's a rare cause of jaw pain but should be considered in the differential diagnosis.
Sjögren's Syndrome: An autoimmune disorder that can cause dry mouth and lead to jaw pain due to decreased saliva production. It's less common but important to recognize due to its systemic implications.