1994 Pontiac Bonneville Voltage fluxations & power loss

Usually when I start car after standing 12 hours, the alternator guage, and head lights flicker. The guage at first stays normal for 30-40 seconds then for the next 2-10 minutes either flickers or just stays on the high end of the red. Then it comes to life fluxating for 2 minutes or so, then goes normal and stays normal. 14+ Also on two occations right after starting, I lost power, only able to run 1,500 rpm for 2 minutes then power back. I have replaced the alternator (on warrenty) even though it tested ok. The same with the battery. No engine codes.