Howdy! SonicWall is actually pretty good. Not like Firewall-1, but still pretty good. I own several, but have also worked on tons of other stuff.
The SonicWall has a log. Have you looked at it?
If you had trouble reaching the firewall inside, I’d suspect virus or worm activity before I’d suspect a DOS (Denial Of Service) – since a DOS is typically applied from the outside.
You didn’t say how many systems you have, but it’s probably not that many.
Get a copy of Ethereal/Windump and monitor the actual traffic – I’ll bet Dollars to Donuts that it will be from a handfull of infected machines.
Also, you might want to invest some time (few dollars) on a machine and run Snort on it. But that would be later. Right now, just get Ethereal/Windump and find those culprits.
p.s. reply privately if you need more help