Have you tried looking at the following registry setting?
HKEY_Current_User\ControlPanel\Notifications\Phone \sleep = 0 ??
If this is set to 1, then your phone will goto sleep.
I had the same issue, been awhile, but this is what I think I think I did.
This keeps the phone from shutting off on a long phone call, it could also shut off the radio if your battery was low?