alternator light and voltage

[derrickmd18 at verizon.net (Derrick flo)]

I have a 87 vw scirocco 16v and when I was driving home last night the battery light came on for a few seconds then went out.  I thought might be because of the cold weather.  then when I was at a drive thru getting food the battery light came on again at idle.  I pressed on the gas pedal and it wouldn't go out.  after about 30 seconds it went out.  when I was driving home from work and speed racing up a hill the light came on and went back off.  

Just to mention I was running a laptop, and head lights when it came on last night.  this morning I was running just the laptop and radio.  then tonight I was racing up a hill I was running a laptop, radio, and headlights.  

I hooked a volt meter up to the car and I'm getting 13.1V max output.  its normally between 12.3 - 12.7v while I'm driving and everything.  Does anyone know what the problem maybe?  I looked at the manual and it said I should be getting 14.1V at 3000 RPM but I'm not.  I drive the car for a hour and 10 mins in the rain to night and I never got a light or anything.