Cardiovascular Syncope: Given the patient's history of transient ischemic attacks (TIAs) and the presence of chest pain and shortness of breath preceding the syncopal episode, a cardiovascular cause such as a cardiac arrhythmia or myocardial infarction leading to syncope is highly plausible.

Orthostatic Hypotension: The patient's symptoms of dizziness and syncope upon standing could be indicative of orthostatic hypotension, especially in an elderly patient who may have decreased blood volume or autonomic dysfunction.
Seizure: Although less common, a seizure could present with loss of consciousness and might be considered given the patient's history of "mini strokes" and the description of the event.
Vasovagal Syncope: This is a common cause of syncope, especially in response to stress or pain, which could have been triggered by the chest pain or headache.

Aortic Dissection: Although less likely, an aortic dissection could present with sudden onset of chest pain, headache, and syncope. Given the potential for high mortality, it is crucial not to miss this diagnosis.
Pulmonary Embolism: The patient's shortness of breath and history of syncope could also suggest a pulmonary embolism, which is a life-threatening condition requiring immediate intervention.
Intracranial Hemorrhage: Given the patient's history of TIAs and the current symptoms of headache and syncope, an intracranial hemorrhage must be considered, as it is a medical emergency.

Subclavian Steal Syndrome: This rare condition, characterized by reversal of blood flow in the vertebral artery due to stenosis or occlusion of the subclavian artery, could lead to syncope, especially with arm exercise, but is less likely given the patient's presentation.
Arrhythmia due to Cardiac Conduction System Disease: Conditions like sick sinus syndrome or AV block could lead to syncope but would be less common and might require specific diagnostic testing to confirm.