power problems

i have a 1993 grand am gt with a 3300 and i just bought it and I am wondering if this sounds normal when i got it I would turn on my headlights, wipers, or when my factory radio hit hard the volts on my gauge would go from about 14 down to about 10 and when i turned my turn signal on every time it flashed the power would flusuate so I took it back they put a new battery in it and it still did the same thing so they put a junk yard altanator in it and it didn't change the problem so now i have bought a brand new on for it and now it goes form about 16 to about 12 can any one out there help with my problem?

Have you tried to change your M.A.P. sensor? I have a 93 that was doing the same thing and all I did is changed that sensor and now it runs top notch