Rep Power: 0

I drive a 94 civic Ex, it has MSD ignition, supercharger, etc. All the good stuff. Anyways just last night as I started my car the battery light indicator turned on. I looked at the voltmeter and it read about 11 volts. So I figured the alternator just went bad. I got the car at 128k and now its at 174k. So its just old, right? So when we left to go back home, my voltmeter was reading various readings from anywhere between 14 volts to 18 volts. The needle mostly stayed in the 18 volt area the whole ride home. The voltage was dropping as I was turing the vent on and off. When it was on, the voltage was in the normal range and when I turned it back off it spiked back up to 18. This morning the voltage remained in the 18 volt area the whole time even while playing with the vent controls. On the ride back just now I was on the freeway and the needle was at 18 but then as I put it in neutral, the RPMs dropped and the voltage dropped a little to about 17 Then when I got off the interstate strangely it read 12 volts even for the remaining duration back to my house. What's going on? I initially thought it was a dead alternator but it seemed to be working fine after last night. It seems like it is overcharging my battery. Any help please, I would really appreciate it. Thanks.

 
Rep Power: 291

i'd say the regulator in the alternator has gone bad or going bad.