Hot and bothered magna

MY 1995 TS Magna V6 has done more than 300,000km and served me well. But on hot days, whether the car has been standing or driven and stopped, it won't start. If you wait and keep turning the key it will eventually start. One garage thought it was the starter solenoid, but that didn't fix the problem. It always starts first thing in the morning or at night. If I leave it in a shady spot, it seems to be OK