It could be your PSU. You seem to have a large load on it. On start-up your computer must run every drive, fan, or device on it which will put a tremendous load on it very abruptly, untill it compensates for the load. Try unplugging your fans and see if the problim persists. If so try to unplug some drives as well. I think you get the idea. You seem to have a lot of fans.
Althoe I must tell you what I think the problim might be. I think this would not be the case, because the problim should occure all the time. But it is worth the try. By the way, how long is a while? and how short of a time period is the computer off?