We're currently running firmware 8.3.3 but have had similar issues with firmware 8.0.1 till now.

As for same switches - no. multiple switches / multiple locations

If I replace the phone, the replacement phone does not have a problem. However the replaced phone repeats it's problem regards of the switch, port, network cable.

We're running CCM 4.2.3 and the phone has no shared lines.

We'll try the 8.3.4SR1 firmware on Monday. I did not see the release notes on this version. What is issues are resolved in it?

Upgraded to 8.3.4SR1 with the same results. The only difference is the screen does not go blank now

What are your switch types? 3550? Do they support the 802.3af poe standard?

All of the switches support POE. They're either 3750s or 3560's. In this particular case, the same port and cabling works on majority of the phones and on other phones it simply freezes.

Are all of the phones the same type, 7961Gs or 7961G-GEs or a mixture?

They're all 7961Gs