More than likely it's the internal regulator going out in your alternator. I have gone through 6. What's more than likely happening is that in that higher rpm band, the regulator has to discharge alot of heat because it's producing alot of power but can only allow 14.4 volts out. So, since it's wearing out, it can't handle this heat and it shuts down. Your batt. light won't come on unless there is Zero or close to it output of the reg.