Type 2 Rotary Drive periodic non-response

[sangaris]

There are several posts similar to this: my Type 2 Rotary Drive works fine for several hours but then fails to respond. After a rest of 10-20 minutes, it works fine again for a while, then drops out again. Chuck's replies have said it is likely a voltage, connection or APC issue. However, when I monitor the voltage to the drive motor when it goes into "rudder response failure" mode there is the same + or - 12.6 volts as when the motor is working. That is, the APC seems to be sending the correct voltage to the motor but the motor won't work.
Is it possible the drive motor is overheating and cutting out?
I was going to send the drive in for servicing but was told it is too old (1992) to be accepted in the repair facility. A replacement is $1500 to $1700 so I'd like to attempt to repair it myself. Perhaps a simple fix like brushes or dirty commutator (it's gone 40,000 miles!).
Does this sound reasonable? Any advice on what to look for?
Thanks in advance for your help.
Regards, Craig Briggs, s/v Sangaris; 46' Amel Santorin