Sierra Wireless Aircard - not receiving data while connected to the network

Question

Hello,
Can anyone please help me out here please.
I have a wireless internet connection on the Bigpond 3g network in Australia. It uses the Sierra Wireless Aircard 880U and the Bigpond connection manager software (version 2.10.18) to connect to the internet. This has been running pretty fine since I installed it some 4 months ago.
Now in the last 3 weeks I've been having connection problems, or actually I'm still connected, but data just suddenly stops coming in. The connection manager software states that I'm still connected to the 3g network; and I can see that data is still sent from my mac (the software tracks the data sent and data received); however there's no more data coming in (or very little, as in bytes), and Safari (or any other application like Skype, the widgets or mail) eventually fails (just happened again: "Safari could not open the page “http://discussions.apple.com/post!post.jspa?forumID=1222&reply=false” because the server is not responding" - while I was still connected according to the connection manager).
Also I can't ping anymore. Packets are sent out, but nothing comes back.
Now I've spent the last 3 weeks on the phone with Bigpond support but they can't resolve the problem. We have tried all the resources; including reinstalling the software, de- and reactivating my account, rebooting, removing and reconnecting the device, creating a specific location for the wireless card as to reduce any possible interference from airport, etc. But this hasn't resolved the problem.
The support people are now stating that it is not their device or software that is faulty (as the can see that I'm connected to the network just fine on their side), but something specific to my Mac that is causing the data problem and that I need to contact mac support.
The annoying fact is that the problem is erratic. Usually when I start up my mac in the morning; I can connect and surf the web without a problem, but later in the day, the internet connection will fail. I will restart the mac and bigpond connection manager and sometimes I may be able to get back online, but most of the time there is no more data (or very little data) coming in even though the connection manager tells me I am connected to the network and data is going out and very little coming back in (a few kilobytes at the most). Personally I have the impression that the Bigpond connection manager software is not too stable, but I can't put my finger on it.
Also the connection manger software seems to behave erratic. Sometimes it is connected and I can surf fine, while it states that it is (still) determining the IP adress, other times it will show a valid IP adress but I'm not getting data (or vice versa). Also I've been getting other error messages, like: "unable to initialise the device" and a mac popup message: " a modem error ocurred; cannot negotiate a valid set of ppp...."
Hopefully theres someone who can give me a hand here based on what I've written. I'm at wits end. Just let me know if I need to supply need more info;
thanks

Answer 1

I have just discovered another aspect of this issue.
If I have the Macbook connected to my router which is then connected to the Mac Pro but the router is not connected to the internet then PS on the Macbook starts up with no problem.
If the Mac Pro is not connected to the Router then PS will not start up on the Mac Book.
I suspect that this behavour started after I installed 10.5.7 - however as I cannot recollect starting up PS in standalone mode prior to last week before I installed 10.5.7 via the combo package.
So any clues please
Derek