1994 Ford F-150 start/battery and idle issues?

I have a 94 F-150 with the 300 straight six. With the key on, the battery will read low. When I start the truck, the battery gauge will read as it should. In some rare cases, most usually when taking off with the headlights on, the belt will scream like Hell and the gauge drops extremely. Usually if I turn the headlights off and drive it for a second the belt will quit and it'll return to normal. If the truck is cold when I first start it, and I try to turn the heat up, it will bog the engine down and kill it. Has a noticeable misfire at idle. In even rarer cases, and it does it more often when under an electrical load, the battery gauge and idle will drop and raise repetitively. I was personally thinking it's a MAP sensor or the IAC. Very occasionally, while driving, I'll hear a whistle type sound coming from what seems to be the engine, which leads to me to believe it may also be a vacuum leak. Any response would be greatly appreciated.

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