92 964 with 65k

Just bought this car from an older couple. A few days ago this car developed a problem of not cranking when it is hot intermittently. This condition usually happened when I have driven the car on the freeway (20-30 minutes),parked it for 5-10 minutes to pickup something. Upon returning and tried to restart the car (Shifter in park or neutral, key position II), all the dash lights came on, I hear a "Click", but the starter will not turn. If I keep trying for the next 15 minutes or so, the starter will eventually crank and the car will start. I have checked or replaced the followings:
1) Battery voltage 12.4v
2) Charging voltage 14.6V
3) Clean and reinstall battery cables and battery ground cable
4) Replaced Starter and Solenoid (1 unit)
5) Replaced Starting Safety Switch (PNP switch)

I have not replace the ignition switch or trace wires for shorts because of limited knowledge on airbag and lack of wiring diagrams. Please help with your knowlege and experience, Andrew.