I agree with Unknown K and Temetka, try to reduce the number of variables and/or things that have changed recently and see if it works better with less extensions, and even less optional cards if you have any inserted. Test the +5V and +12V at the disk connector with a voltmeter, unplug and replug all power connectors also. Even if you cannot see any wet around the motherboard capacitors, remember that the SMT types will start to leak directly underneath first before spreading. Should there be any surface traces running under the cap they will start getting random and growing connections to others in the vicinity, with symptoms that change the longer power is applied, and also, at least in the beginning, symptoms that can temporarily go away with an overnight power off! If you have the equipment, my favorite temporary trick is to ground the tip of a shopvac crevice tool with some foil and a ground wire, and use a squeeze/squirt bottle of denatured alcohol to spray/suction each SMT capacitor base, then let dry for 24 hours just to be sure before testing again. This is not a fix, if there is leakage the caps all should be changed.