Intermittent problems are real frustrating because there is no defect to be found when the problem is not occurring. Next time this happens, measure the battery voltage while the engine is still running, then measure the voltage on the generator's output terminal. Those two should be very nearly the same. If the output terminal is a lot higher, there's a break in the circuit going from there back to the battery. Look for a large fuse bolted into the under-hood fuse box and be sure those nuts are tight.