Alternator problem 95 Nissan Pick up

Truck quits running, alternator no good, replaced with 2 rebuilt and 1 new, still won't charge on idle, it will charge when first started then after high idle , it won't charge, I have a new battery, 2 weeks ago. Could a defective idle control valve have any thing to do with this problem. My mechanic has done about every test he can think of. Truck runs great when battery has full charge, and has no problem starting. Please if anyone has information I would appreciate it. Thanks

6 cyl.