Start by having the alternator professionally load-tested. The two things to pay particular attention to are the maximum output current and "ripple". When one of the six internal diodes fails, the most output you will be able to get is exactly one third of the alternator's rated output of around 90 - 110 amps. When that happens, ripple will also be very high. Ripple is the rapid variation in output voltage. The voltage regulator responds to those changes in voltage and tries to correct for them. The result is often flickering lights.