asterisk users - Jun 2005 - Max Retries Exceeded - IAX2. Network problem?

Hi

We're having some problems with max retries exceeded errors using IAX2 which
causes dropped calls. Sometimes happens with Firefly softphone, now 1.9.9 (the 
current one) but has also happened with a hardphone we use (IAXtel). This is 
just for the internal connection between our desktops and our switch - these 
are calls which then go out ver ISDN/PSTN and the error is definitely an iax 
channel error, which means internal.

So my guess is now that this is due to either a problem with network 
connectivity - our switch or with the network card in the Asterisk server. 
There is some suggestion it happens when the network is busy.

Can anyone suggest a way of checking this out? I think I'm going to buy a 
different switch type and see if ti has an effect but a more systematic way 
would be better.

Thanks

Paul