When I had a similar problem I wondered about the B&C alternator but found the batteries were the problem.  I had checked the voltage at rest and everything seemed reasonable but in flight there were electrical failures and problems.   The A&P put the batteries on a load test and they failed immediately.  New batteries=no more problems.  So, a good annual check is load testing the batteries just to make sure they can handle the loads from hyd pumps and other high-amperage draws.