Intermittent Startup

I have this intermittent computer startup problem
 When the computer fails to fire up the monitor displays a ?NO SIGNAL? notice A tap on the space bar starts it off. It will continue to ?DETECTING IDE PRIMARY SLAVE? where it stops, a further press on the space bar will allow the computer to get into Windows
 It?s seems temperature related ie. When the temperature is warm it usually fires up OK
 It?s been a problem for about 12 months and is now more prevalent
 My serviceman can?t find the cause
 I have checked the motherboard?s plug in items to make sure they were well seated
 Any help will be appreciated