I have some 7945's that register to CUCM 8.5.x.
The reg fine and pc behind commicates without issue.
Strange thing is after a certain amount of time, the PC seems to lose connectivity after a few hours or few days.
The PC was and is fine without the 7945 inline.
Using Brocade FCX648_HPOE
Strange thing is I have 6941's working fine at that site and have replaced the 7945's with multiple new known working phones.
Is it happing only with Cisco 7945 Phones or all the phones in your organization? If its with only 7945 Model phones try factory default setting on the phone