voltage regulator

i have a 1999 hyundai tiburon that is driving me nuts. I have been through almost $1000 fixing parts on this car and taking it to idiots that can't tell me what's wrong or just charge me to replace something else. Anyways, it has appr. 95,000 highway miles on it, it is a standard tran., two wheel drive GT 4 clinder. When driving the car at night, the headlights suddenly get really bright and you can hear the car motor surge almost. It does it without the headlights on too, but it is really noticable at night. It slows my car down some too, almost as to lose power. One guy said it was the flex pipe, $200-not it. Another said tune-up (which I do on time anyways) -$200-not it. The other guy said fuel filter $30-not it! The list goes on. Now someone is saying it is my voltage regulator (not to mention I have had my alternator tested and it was "fine"). So that is gonna be another $200. My main question to you is do you think this is what it is, can I just replace the voltage regulator, or what could possibly be wrong? (already go a new battery too!) Please help! Men at shops here suck and of course that is probably because i am a naive female! thanks