Meralgia Paresthetica: This condition is characterized by sharp, shooting, or burning pain in the upper inner thigh due to compression of the lateral femoral cutaneous nerve. The symptoms described, such as sudden onset, short duration, and occurrence on one leg or the other, are consistent with meralgia paresthetica. The fact that position does not matter and there are no skin changes also supports this diagnosis.

Neuropathic Pain: Conditions like diabetic neuropathy or peripheral neuropathy could cause sharp, shooting pains in the thighs. However, these conditions typically have a more widespread distribution of pain and are accompanied by other symptoms such as numbness or tingling.
Musculoskeletal Issues: Strains or tears in the muscles or tendons of the upper thigh could cause sharp pains, but these are usually related to activity or position and might be expected to have some associated swelling or skin changes.
Referred Pain: Pain from the lower back or hip joint could be referred to the upper inner thigh. Conditions like lumbar radiculopathy or osteoarthritis of the hip could cause such referred pain, but typically, there would be other symptoms like back pain or limited mobility of the hip.

Abdominal Aortic Aneurysm (AAA): Although AAA typically presents with severe, tearing abdominal pain, in some cases, it can cause referred pain to the thighs. Given the potential lethality of a ruptured AAA, it's crucial to consider this diagnosis, especially in older men.
Deep Vein Thrombosis (DVT): DVT can cause pain in the thigh, although it's usually more constant and accompanied by swelling, redness, and warmth of the affected leg. The intermittent nature of the pain described makes DVT less likely, but it's a condition that cannot be missed due to its potential for severe complications like pulmonary embolism.

Nerve Root Compression: Compression of nerve roots in the lumbar spine (e.g., L2 or L3) could cause pain in the upper inner thigh. This would typically be associated with other symptoms like back pain, sciatica, or changes in bladder and bowel function.
Pudendal Neuralgia: This is a rare condition characterized by pain in the distribution of the pudendal nerve, which could include the upper inner thigh. The pain is often described as burning and is typically exacerbated by sitting.
Sickle Cell Crisis: In patients with sickle cell disease, a crisis can cause severe pain episodes, including pain in the thighs. However, this would be accompanied by other systemic symptoms and is less likely in an older man without a known history of sickle cell disease.