[blucar]

Back in "the day" when it was common to overhaul and engine, utilizing limited machine work experience/facitities, i.e. seat of the pants shade tree fitment, it was very common for the overhauled engine to be very hard starting when the engine was well warmed up.
An overhauled engine was generally ran for 500 miles using a light weight break-in oil at speeds not to exceed 35-40 mph. The break-in oil was then drained and refilled with 30w oil. This whole procedure was repeated when the engine had 1,000/1,500 miles, with the maximum speed limited to 45-50 mph..
I would submit that the problem herein is actually and engine that is set up a little tight. The problem will resolve it's self in a few miles.
In 1954 my wife to be had a '46 Studebaker Champ that blew a rod.. I pulled the engine, had it rebuilt by a local shop. I re-installed the engine, it ran great, however, after the engine warmed up it would not crank over.. Let the car sit for an hour or so, it would start right up.. My girl fried needed the car to get to work. It would start right up in the morning and in the evening at the end of the work day.. If she stopped on her way home from work the car would not start under it's own power..
After several weeks of being stranded my girl friend was at her wits end.. I finally traded cars with her. I drove the Stude to work for weeks, it slowly got better. We resorted to taking the car for long weekend drives.. By the time the engine had close to 4,000 miles, it would start right up hot or cold..
The problem here-in could be one of those.. If it ain't broke, don't fix it deals..