I just got my Jetta running. I have a similar issue. I would say that I get about 1.5 to 2 pulses a second. I've checked the voltages, and they fluctuate to 12.5 (off pulse) to 14.1 (on pulse). You can see the tach drop 100 RPM when the alternator loads, then it goes back up when the alt. unloads. I think I'm going to replace the voltage regulator and see what happens.
I know that you can replace the regulator without pulling the alt, but how on earth does the alt. come out? I got my car with a bad engine, and as I was doing the swap, I tried to pull the alt. in frame, and couldn't figure out how it comes out, so I left it in and swapped it on the stand before putting the new engine in.