I have a 97 Mustang and the guy who tuned the computer said sometimes altemators develop a "dead spot" at idle RPM, whereas it won't put out the juice it should at idle. Is this a line of BS, or has someone heard of this before? When I take it into a shop and they spin test it it reads OK, but I don't know what RPM they spin it at.

Hey Vids,
If you have a under drive crank pulley on your car this would be a likely concern.

If it is stock then suspect high resistance in your positive battery cables affecting the battery sense ability of the alternator. Do a loaded voltage drop test of your battery cables. Start between the battery post at the alternator and the positive battery terminal. If there is more than 0.5V drop then check between the positive battery terminal and the inlet side of the alternator fuse (20A in the engine comp fuse box). If nothing is found there then check between the positive terminal at the alternator and the alternator fuse. Still looking for less than 0.5V drop. If you have high resistance you will most likely have to replace the cables unless you can find/repair the source of the concern.

let me know if this makes sense