Power up

hi and thank u for letting me join,i have a emachine e725 (acer) which has a mind ov its own, every thing works when it does turn on, if i keep pushing on button for about half a hour will boot,problem is..... on turn on all lights come on but no hard drive light, fans ect work but then shuts down after a few minites, have removed both ram boards turned on, with no bleeps ?. cleaned them and put them back the same, also the same wen not in but like i said,, if i keep turning on and off it will all run ok, any help i would be very gratefull, thank you.