Alternator, Voltage Regulator, or Something Else? Options

[yeahmag]

I'm having an intermittent problem where over 3500 RPM (cruise speed for example) the alternator light will come on. If you LOWER the RPM the light goes out and has never come on at idle even with a full electrical load. I've done the following to try and figure it out:

• I've swapped the VR once (I have a few more to try)
• I've checked voltage at the battery while at idle and running in the driveway
• I've checked for AC voltage at the battery, which as I understand it would indicate a bad diode.
• Checked belt tension

I plan on checking:

• Battery cable at starter
• Ground strap off transmission
• Verify the relay board is sound below the VR

Can anyone think of something I may be missing here? I know my Optima Red Top is getting old, but I can't see how that would fail in this way. Got a race on Saturday and need this sorted! Help!