1) Change out the phone to another one and program the new phone with the old phones DN. See if problem persists.
2) Put problem phone on a known-good port on the switch and see if the problem follows the phone.
If it is the port then patch the drop into a different port on the switch and make any needed changed on the switch (VLAN, etc)
3) You can also put a power supply on the phone (if you have one) since you are using POE and see if it still reboots. If it no longer reboots but just looses connectivity then this points to the switch as the culprit.
These steps should get you some quick and dirty answers.