I have had exactly the same problem with an RB750, and a pair of RB751s. I do not use PoE injectors but use the external power supply.
What external power supply are you using?
I have been running a test for a couple years now, monitoring grid tie voltage vs output of an unregulated power supply. This is a 12v 1A unregulated wall wart connected to a RB433AH. I have verified the voltage reading is correct within one tenth of a volt with my meter. This is the last day's report from a three day run.
Daily voltage report for test on aug/10/2012
23:00 = 14.7 Line voltage = 124
22:00 = 14.4 Line voltage = 122
21:00 = 14.2 Line voltage = 120
20:00 = 13.9 Line voltage = 118
19:00 = 14.1 Line voltage = 119
18:00 = 14.0 Line voltage = 119
17:00 = 13.9 Line voltage = 118
16:00 = 14.1 Line voltage = 119
15:00 = 14.1 Line voltage = 119
14:00 = 13.9 Line voltage = 118
13:00 = 14.0 Line voltage = 119
12:00 = 14.4 Line voltage = 122
11:00 = 14.0 Line voltage = 119
10:00 = 14.4 Line voltage = 122
09:00 = 14.2 Line voltage = 120
08:00 = 14.4 Line voltage = 122
07:00 = 14.4 Line voltage = 122
06:00 = 14.6 Line voltage = 124
05:00 = 14.9 Line voltage = 126
04:00 = 14.9 Line voltage = 126
03:00 = 14.9 Line voltage = 126
02:00 = 14.8 Line voltage = 125
01:00 = 14.9 Line voltage = 126
00:00 = 14.7 Line voltage = 124
Since voltmonitor started on 04:00:10 aug/07/2012
Maximum = 15.3v at 04:00:10 aug/08/2012
Minimum = 13.6v at 20:00:10 aug/09/2012
If this is 12 volt unregulated, then multiply those readings by 2 for a 24v unregulated supply. Bear in mind the maximum input voltage to those devices is 28vdc. How many of those readings would be over 28v?
edit: My test is to plug in the wall wart while not connected to anything. Measure the output voltage. If it is near or above 28 volts,
don't use it!