computer wont boot, then it will??

:confusedacer aspire desktop t650 running xp, started not booting, fans run but nothing else happens, after turning it on/off several times hey presto boots up fine, that was going on for about 6 months then just would not boot at all. so took out graphics card and computer booted up fine for a week ..then same problem . i have to turn it on/off and hope it boots i keep it running all the time now, if i have to restart it shuts down and starts up again fine so it seems weird. anyone have any idea what it could be?
if something had *gone* in it it wouldn't work at all would it? but this works when it wants to