Arduino UNO R3 Resets itself

I have an Arduino Uno R3, and it can run sketches fine other than it resets itself every 10-30 seconds for no reason.

I have checked the board for any visual problems such as cracked components or foreign objects, but the board is clean and I can't seem to find any problem with it other than half the time when I try to upload a sketch it says "Serial port 'COM5' already in use. Try quitting any programs that may be using it."

So far I havn't checked if it works fine on battery power.

As you could imagine, having your arduino reset every 10-30 seconds for no reason can be very annoying..

it can run sketches fine other than it resets itself every 10-30 seconds

With nothing connected to the board?
Is anything getting hot?
Is there static electricity around?
Is it resetting all the time just sitting and running a code like blink?

Thanks for a quick reply.

Nothing connected but the USB cable to my computer.
Nothings getting hot.
It's a cold and damp day, there doesn't appear to be static.
Running the blink code, it blinks like it should for around 10-30 seconds, and then resets. Same with a blank code.

Extra info: It does blink like it should when it resets, and no I havn't tried on external power yet.

Does it, blink like it should when it resets, blink like it should when it resets etc. ?

Yeh, it runs fine.... Reset. Runs fine....Reset etc

And I timed how long it worked until it reset. Each time I timed it was different. So I'm assuming it's not having some sort of code overload, where you would think it would be at least consistant

UPDATE

It works fine on battery power. I am therefore asssuming that it's my computers fault (or the USB cable).

You might have some kind of USB dock syncing software that may be polling the Arduino and causing it to reset.