i think your problems are related, sounds like a vacuum leak, maybe from intake manifold gaskets. as your engines rpm declines the alt output goes way down and causes that other junk. i would see about the misfire then if it is separate go from there. you have any white stuff in the oil, or rad cap? sometimes you can check for vacuum leak by spraying around stuff (cyl 4 area especially in your case) with carb or maf cleaner, listen for engine to change pitch. that engine has known issues with the gaskets and i think felpro has a updated set for the job.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock