problem
 

my volt meter is reading 12.5 volts and is miss firing alot. does my alternator is is probably bad or is something elses? thanks

hmmm,go down by the starter and check the ground,also check it by the coils.

watch your alternator belt for slippage.
try to get a reading right from your alternater.
pos is the big nut on the top of it.
if none of this helps,you need a new alternator.

Why is 12.5 volts that bad? Sounds pretty normal to me - maybe a little on the low side.

I think you might just have a grounding problem. Try running a second grounding cable to a bolt on the engine or whatever, see if that solves the problem. BTW my volts used to waver at 12-13, and the second ground brought it up to 14 and rock solid.