Sounds like it's your main relay. I had a similar problem, replaced my relay, and have not had the same problem since.
Check out these links. They show you how to fix it if you have soldering skills.
http://www.marklamond.co.uk/howto/el...main-relay.htm (link fixed)
http://techauto.tripod.com/mainrelay.htm
What happens is the solder joints dry out and separate. The with the heat of a summer day, the joints then expand and contract causing intermittent starting problems.
The main relay is not a pain to get to, and is easy to take out and put back in.