I was sitting at the stop light last night with my headlights on and my radio blasting, after sitting at the light for a moment my engine kicked down, and my voltage droped to about 10-11 volts. Enough of a drop to dim my headlights and for my radio to turn off from lack of power. My car only seems to do this when it has been running for a bit. Cold it will maintain 13-14 volts all the time. I recently replaced my alternator and battery. Any ideas of what this is and where to look to fix it would be great, Thanks.

 

Do you have underdrive pulleys?

 

No I don't have underdrive pulleys, my car is stock except for the K&N filters, which I am sure have nothing to do with this problem.