After the simple tests suggested: It shifts fine when everything is cooled down, engine running or not....so I'm guessing that when things heat up, it changes the clearance slightly....maybe a slightly warped disc/pressure plate; maybe things not sliding on shafts properly when hot and/or pedal clearance needs tweaking to give a little less free play? It's interesting that the problem was only with first gear....even when hot, it went thru all other gears just fine.
