[Turnkeys]

I've had this issue pretty much since upgrading to the stock 3.35 ROM. For me, any app (and/or today plug-in) that 'grabs' for internet access can cause this. If the phone's asleep, as soon as it's turned on or a call wakes it, the offending app grabs a data connection to update, and the radio dosen't release it fast enough to answer the call.
The default timeout for an idle data connection is 10 mins. If you're checking every 5, it's likely that you're constantly connected.

I'm hopeing the new ROM/Radio address this, but I'm waiting.

Mean time, I've disabled "as new mail arrives" on my Mail2Web exchange account. I have GPS Today, but set it never to connect automatically. The only other items on my today screen are SPB time, and a radio (BT/Phone/WiFi) toggler that I forget the name of... also check your startup folder for anything that autoloads and updates.

I installed SPB's GPRS Monitor a while back 'cause I was curious of my data usage, left it installed as it has a handy 'connect' button on the status screen.

Hope this helps.
Crossing my fingers for 3.59