Dell Studio XPS 8100 not turning on

Having rather strange problem with my desktop, if I open up the case to add new hardware or even just have a look when I set it back in my desk and plug it all in it will refuse to turn on in the upright position just a flicker of light from the power button. Usually I fixed this by taking out the new hardware or just unpluging and opening up the case for a few minutes before pluging it back in and it would turn on as normal after being plugged back in. This time however I opened it up to add a hard drive and then realised i didn't need to so i just closed it up and plugged it in, trued to boot it up but as usual it refused on the first try. So i tried the usual method of letting it breathe but that didn't work so i decided to try and boot itup lying on its side, to my surprise it booted up fine, so i thought great it's working again, put it back in the desk in the upright position and it won't turn on. Had a look about and noone else seems to have this problem so i have no clue how to fix it. Any help at all would be appreciated.