Charging Woes

I own a 1997 Nissan P/U and am having problems with flickering brake and battery lights at high RPM. I replaced the battery and grounding cable but to no avail. I've been researching on the WWW and most have answered similiar problems saying it's the alternater. I've had the alternater tested and it checked out fine. I found during this research that the battery light comes on when it's running on battery power and that Nissan made the brake and battery light come on similtaneously to indicate a charging problem. So then I thought maybe since my problems never start below 3000 rpm and the auto shop runs their test at 2000 rpm maybe they didn't catch it. Could that be possible? Any help would be appreciated I'd hate to buy an alternater for nothing. Thanks in advance,
Brian

Brian
I don't know who tested your alternator but I have seen this at Advance Auto with my own eyes. I took an alternator in there to be tested the guy hooks it up on the machine and it tests bad which was kinda of a surprise to me because I had hooked it up to my little electronic gismo tester before I took it out there and it said it was good but I wanted a second opinion. So me and this guy gets to talking about this and that and I tell him about I had already tested my alternator and h said let me show you something. He took two alternators off the shelf one new one rebuilt and tested them the new one barley made it to the passing point and the rebuilt one showed bad. He said it happens all the time the machine is about wore out or the leads are frayed or some of the workers just don't know how to hook it up. So I took mine to my rebuild man that does my work told me I had a diode going out and my bearing was going so it was about time for it to fail.