If the phones stop rebooting by firmware change then the issue may be with the specific firmware, unless you are saying issue is with certain 7821 and not all. Did you try checking any logs etc. - by browsing to the GUI or from the phone settings by checking status messages:
Ref: http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cuipph/7821_7841_7861/10_1/english/admin_guide/PA2D_BK_AB3F74DA_00_admin-7821-7841-7861-10_0/PA2D_BK_AB3F74DA_00_admin-7821-7841-7861-10_0_chapter_01100.html#PA2D_RF_S6F1CD83_00
Also check the logs etc. on the switch. If issue is with only certain phones and not all 7821 then try establishing anything in common with the phones rebooting, same floor, same patch panel, same switch.
-Terry
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