Found the problem! But how do I fix it?
 

I FINALLY found what is causing my car to act crazy...

Anytime I turn on my headlights, rear defroster, or a/c compressor, the idle on my car goes from a very smooth 900 RPMs to an extremely rough idle until it finally dies. While driving, if I don't turn any of these on, it's perfect. However, once I turn them on, it has a huge loss of power until it finally dies.

So that's what causes it, but I'm at a total loss as to how to fix it? Any help would be gratefully appreciated.

On a personal note, I come from a background of Lotus cars. Most recently, I tracked an Exige from 2006 to early this year, but decided to stop being a slave to consumer debt... I sold the Exige and decided to "make do" with a 944 until I had enough cash for another Exige. But I've got to say—I love this car. On top of that, this community is great, and I've really enjoyed going through the forum and learning all about these cars. Well done everyone. The 944S will definitely be a permanent to my garage. SmileWavy

Hmm. How does the alt test out? Might also check the quality of the connections on the battery. I know it sounds simple but hey.

Yeah. Check the battery clamps, their connection with the cables, the ground connections at both the chassis and the back of the engine on the top edge of the clutch bellhousing, the positive connection at the starter (since the alternator is also connected here and that's where the battery gets its charge from), and check the alternator connections. Also, the large cables between the alternator, starter and battery can corrode internally, reducing the current carrying capability. If everything else checks out, they may just need to be replaced.

Sounds like your cabling from the battery to the fuse panel and/or alternator is damaged and not allowing enough power to flow through the fuse panel to feed all the devices.

Check your grounds....

Thanks everyone! I'll check all this stuff out between now and the weekend and let you know.

+1, alt output or wiring (including grounds).

+1