I am betting you are actually hitting dishnetworks satellite. When I helped a friend point his, we would only get one set of transponders because we were actually hitting 110 or 119. The DTV box showed it as hitting 101 though.
That would not explain the constant 21.2 voltage (it should be 13v or 18v), and he tested the voltage at the LNB, so I assume that was done on the other side of the switch (except the internal one if a at9 or phase3).
IMO, Jimbo hit it on the head, imho, it's most likely a LNB, or internal switch (I think a 4x4 in the phase3 and a 5(8))x4 in the at9??) issue, just remember, if you use a phase3 or at9, you have a switch built into the dish, so if you use an external switch in addition for either of those 2 dishes, and bypassing the external switch as Jimbo suggested does not help, it still could be the dishes internal switch, switching out the LNB like Jimbo said will indicate that it could be the internal switch.
It is also possible it is a receiver problem, as the OP indicated it's an old receiver, perhaps it went bad, an old capacitor or resistor could explain a abnormal voltage, and could cause switching to malfunction.
It would be in the OPs best interest to try and find someone who has D* and try his receiver on their coax, hoping the receiver, if it has a problem, does not damage the friends setup.