Need somebody smarter than me

[cordbob]

Friend of mine has a 48 Mercury stock engine, drive only a mile or so when it warms up go to restart it starter relay clicks starter is drawing amperage does not rotate engine at all.Checked cables and ground connections.Jumped power directly to starter terminal , installed new starter same result.If the car sits for couple hours fires right up.Load tested battery reads o.k I'm stumped engine runs fine drives great?