|If only 1 second at a time how do you know the computer is starting at all. Describe all you see and hear.|
Have you opened the case to look? There are many reasons that could happen. You need to troubleshoot a bit to narrow down the possible causes.
With the case open first look at the fans to see if they are all gummed up with dust, dog hair, etc. If so, blow out the case using canned compressed air. Do not use a vacuum. You can damage sensitive components from ESD.
Next, while attempting to start, watch for fan movement and lights on the motherboard.