A male patient presents with intermittent episodes of paroxysmal weakness for 2 months, relieved by rest. Recently, he experienced another episode upon waking after alcohol consumption, accompanied by fatigue. Physical exam: alert, BP 140/80 mmHg, no abnormal lung findings, heart rate 98 bpm, lower limb muscle strength grade 3. Laboratory tests reveal serum potassium (K) 2.16 mmol/L. The most likely diagnosis is: