2001 Infiniti I30 intermittently will not go into 2nd when outside temperature is hot

I drive a 2001 Infiniti I30 with about 160,000 miles. About 3 years ago I began to have a problem. Intermittently my car will not go from 1st gear into second. It has an automatic transmission. The problem began in August 2007. I took it to a transmission specialist. He couldn’t determine the cause and asked me to bring it back when it failed. When it failed again I took it to another specialist. He looked at the transmission and said everything looked OK. During the winter everything works fine. When the temperature begins to warm up, it starts to fail. If I pull over, turn off the ignition completely (leaving the radio on doesn’t seem to work) it will restart and go from 1st to second. On one occasion it failed while traveling at 60+ mph on the freeway. The temperature here has been in excess of 90 for 17 days. It is now failing regularly.

Any ideas on the cause and the solution?