a bad ground can cause all these symptoms. It may be bad battery terminal or the ground on the harness from the battery could be making a intermittent connection. I've had quite a few military vehicles show these symptoms, Tracked the grounds down and cleaned and checked the connections up and it solved most of them.
Also in some cases the battery's showed good voltage but had almost no amperage. Check the complete output of the battery, voltage and amperage. This can cause some issues although unlikely in your case.
