dm500 keeps crashing

bit more info,how old is it? how are you flashing it? what images are you trying?

If you're using dreamup and null modem,then you should'nt get any probs.
The only thing i can think of is if its crashing after an hour or so,is it over heating/component failure and on its way out?

Not unless you want to research swapping the capacitors and looking for dry solder joints.
Just do the obvious first tho and make sure all connections to the dreambox are fine and that the power supply is fine(try swapping for another 1 and see if you get the same result)
But if you find that it is overheating/board/component failure,then its not really cost effective to be messing around with for the sake of 45-55 quid for a new one.

Put a fan under/over it..even lift it up off the surface ,,leave it on and monitor it ..wil post pics to add an internal fan later if you want ;)

at last , someone has the same problem as i had.. thought it was just me,, madbriuk, i had same problem, sick of it really, for a finish i flashed it so many times, box wont work any more.. nothing got to do with fan , or cooler or anything its just unlucky we got similar boxes.. i even sent my one back, guy didnt even look at it, just put it down for tuner problem.. man i feel your pain..

These are some images I found on the net :
Both have implemented ON/OFF switches

the cable is a bit thick in this one(thinner might overheat quicker) to add an ON/OFF switch

credit to tonimoreno
or alternatively:

credit to yosimura

alternatively if you dont want to solder : then look for a laptop fan with a usb charger

In both examples you can see that a switch is used for manual ON/off if the need arises thats all ;)
Glad you got it sorted either ways ;)