1989 Mazda 626 unable to start engine

I am unable to start the engine on occasion. When the engine is running, it runs very well. However, sometimes the enging will not start. I leave it for about 15 minutes and then it will start. This is happening more frequentily lately. I have checked the spark when cranking the engine over and it has a strong spark. This is the 2.2 ltr. Efi engine with auto trans. When the engine does start, it doesn`t blow black smoke, cough or splutter or idle or run rough. I recently took it to my local Mazda dealer workshop they checked the fuel pressure, timing and a few other things but couldn`t find the fault. The engine usually starts straight away, but sometimes has a very high idle speed, around 1500 rpm and stays high until the engine is at operating temp.