I had the exact same problem...Turned out all it was is the belt slipping. The GMs are known for this "pulsing lights" problem. The voltage regulator turned out to be the most common culprit of this.
If the belt is good and tight, my bet is you won't waste your money on a new regulator, but go for a quality one, not an aftermarket. It'* been my experiances that most of the aftermarket ones will fail and make you spend more money only to find out you had a faulty regulator to begin with!