Jczar,
A little more information would help the diagnosis process. What engine are we talking about here, and is it an old, high mileage beater or a recently rebuilt engine? Is it stock or modified, and if modified what are the highlights of the build? Have you checked your ground connections? Your starter may just be weak, able to crank the loose cold engine, but unable to overcome the added friction of things heated up. From what you have said I see no reason to suspect the regulator - it's part of the charging system and you say it checks OK. I did have an old, high mileage 348 chevy as a kid and I went through a couple of starters and batteries chasing an identical hot start problem which I later found was being caused by lots of carbon in the rings expanding when hot. It solved itself by breaking a bunch of oil rings on a high rpm romp one night....