David - I'm watching this one closely, as I've had a similar phenomenon. She always fires right up cold, and usually fires right up warm, but occasionally, she takes a long while to start warm. If/when I get her going during an episode, she doesn't want to keep going. The A/F is rich during this time. There definitely is something intermittently off with the fuel delivery. If I leave her sitting a while, she'll fire right up again and runs normally

I've been thinking fuel pressure regulator also.... (I have fresh MAFs, and this tendency preceded a recent MAF swap.)
Dave