If it starts fine cold it has plenty of compression to be a runner. Valves will tend to loose their seal when hot as things expand and move during the heating cycle. The engine will run as long as you keep it going. Then when you shut it off they can be a bugger to restart, until they cool. Also verify the exhaust valve guide is not overly loose, old engines have issues with that too. Pop out the valves and look at the face of the exhaust valve. If it is all black on the sealing area it is leaking. At minimum lap the valve (cutting the seat and face is far better) and then set the valve lash. If you work fast it takes less than an hour to do this job.