No monitor on start up

Not sure if this is the correct group - it feels the closest.
the problem
Sometimes on start up, the screens (I have a 2 monitor set up) remain blank. I have to restar or close down and start up, perhaps two or three times before a normal boot comes up on screen.
3 possible things come to mind as causing this.
1 I thought I'd do a gentle bit of overclocking (since reset back)
2 USB ports, both 2 and 3 are my personal nemesis. I loathe them because of the appallingly poor design. Occasionally I try to insert a USB cable into the floor mounted PC, get it the wrong way round, and although it won't fit, of course, it can make an incorrect contact - which invariably forces a shut down.
Either of those two issues makes me suspicious that they are related to the fault, which occurs intermittently. Sometimes start up is quite normal, sometimes I just get a blank display, which only 'cures itself after a few attempts at start up.
Anyone had this issue?

Re the usb point - You're right of course - I never fully inserted it. But just putting it against the socket triggered some sort of 'short'. The 2 outer contacts on the 'plug' aren't recessed much. As you can see, they are very near the edge, perhaps close enough to make contact if you try and insert the plug the wrong way.
Re Graphics card and monitor - I did all that. In fact 3 months ago I spent about 3 hours vacuuming/blowing dust out of this desktop. It was clogged beyond belief and sounding more than a little noisy. It's whisper quiet now. I had to disconnect quite a few cables to achieve total access and took the opportunity to reseat memory, graphics card - everything I could lay my hands on. I've thought that this could be the cause of this problem but at least 2 months has elapsed since the clean up and it's had near constant use since then so I don't think the problem relates to that.
Needless to say, it's not manifested itself since posting

That's a brilliant link, thanks. Still a lousy design to my mind, considering just how much space and length there is inside. A bit like some HDMI cables where the weight of the cable puts an unnecessary strain on the plug. Optic fibre even worse - I've bought a cheap, thin Chinese job for the home theatre set up. Works a treat and don't have to support the weight of the old cable now which I had to do previously.
PC still behaving itself. If i didn't know better I'd suspect gremlins. hate it when I can't identify a fault I've spent hours looking for!

I use those 90 degree adaptors to semi prevent the stress that's plain to see on some units - but it does mean introducing 2 more interfaces where ideally the fewer there are, the better. I've long ago concluded that in many areas technology has actually gone backwards.