Same problem here. Its been ongoing for several months now, however as you say, Overture support are not interested. One of these days they are actually going to return a call. I just wish they were as professional as MSN Adcenter support who actually seem to want to help you and contact you on a regular basis until the problem is resolved.
Until this problem started to occur I was spending $2000 each month with them - its down to about $200 now with the residue going to Adcenter.
The last time I spoke to someone they said it was my firewall, however it still occurred after the firewall was switched off so I don't know what to do now.
Tried that, didnt work. I've even tried it on a neighbours pc. Still doesnt work. I've been on the phone again to support who refuse to give me an email address to someone who I can escalate this to. The frustrating thing is that I have $700 in my prepaid advertising account that I cannot use or withdraw.