If it's still in Warranty they'll replace it.
I have the exact same problem on my DG834PN and yes a reboot does sort it but then it's only a matter of time before the cycle repeats.
I've gone through all the stuff their tech support said to do, I actually did all of it before I contacted them...
In the end they said we'll replace it...so I'm just waiting for the stickers to come through!
One thing you don't seem to have done it to reset the router, get a pin and stick it in the hole in the back of the router for about a minute...
Up to you.