Recently my '96 Ranger developed a charging/electrical issue. All gauges read fine on startup & low speed travel. Anytime the engine RPM's go over approx. 2500, my battery light comes on and the charge indicator drops back to just above the "L" (normally its at about 55%). Battery posts look fine--no corrosion. Battery was replaced within the last 12 months, alternator was replaced about 5 yrs back. I've already removed the alternator & had it bench tested at Advance---it checked out fine---but I'm not sure at what RPM the bench test is done. Any ideas would be appreciated.