internet/wan drop out

Hi,

I have a bizarre situation happening on my network.  We have 5 Dell powerconnect 6224 poe gigabit switches, running with a mitel phonesystem, a managed pacnet mpls router for wan, and sonicwall 2400.  Basically what is happening it the the internet randomly drops out for 3 minutes, 10-15 seconds or sometimes never comes back.  Also sometimes the wan link drops out.  To clear this a reboot of the sonicwall fixes it, but then we get drop outs of 10-15 seconds.

We have tried new sonicwall, latest firmware, got sonicwall to check the rules and they say every thing is ok.  One thing that bothers me is that there is no layer3 routing setup and everyones default gateway is the sonicwall.  one time we left the internet down to fix the problem on new firewall, and switched cat 5 leads back to previous firewall that was running ok at the time and this had the same problem.  after 20 minutes the internet came back.  We have checked with ISP and they say nothing is wrong, done multiple tests with them over 3 days.

i have a sys logger on running to the sonicwall and during the first drop out there is just a gap in the logs, also i am runnning ptrg on the lan but nothing really stands out.  Also the sonicwall has max number of 32000 connections and its peak has only ever been 2400.  there are about 120 users.  also the internet link is a 8mb leased line, and we have a 2mb fail over line that flicks over when 7.5mb is used up.  I have tried to recreate the problem by downloading 2 x 3gb iso but the link stays up but slow.

a colleague of mine suggested it maybe skype and supernodes problem. I am not sure about this.

the only thing i have not done is update firmware on dells, or changed config for layer3 routing on the switches, which is a major job.

Any suggestions please help.  I thing it is the dell powerconnect that is doing dodgy things with traffic, but it does not explain why the firewall reboots it self.  Or have i got multiple problems here?

Thanks

Phil