If your alternator is always reading 13V then it sounds like your alternator is ok. Do check the connections at the back however.

I reckon it could be a clogged cat - it wouldn't hurt checking. I'd also check all connections involved in ignition, and all grounds! Undo the grounds, file back the metal til its clean and rebolt them on.

Good luck,

 

Yeah i just took off my cat right now...and well...its NOT clogged...NOTHING IS IN THERE actually...so ok its not my converter (at least i dont have to buy a new one)....hmm now to try around the ignition area.......

 

A cat will not cause your tach to be jumpy. Do worry about the cat. As for your problem with the tach, mine just started to do it this morning. It scared the crap out of me when I was running 80 because the tach jumped to 9k and the stupid buzzer went off. When your half asleep at 6:05am driving home that will get ya.

Any how, I would check all your connections. Pull off your ignitors and ohm them out. If you have a bad one replace it. Also to be on the safe side pull off the alt and run it down to a part store to test it. Also it would not hurt to add an extra ground.
Due to the temps out side and me being sick already my GSL-SE will sit til I can get to her.

 

Yeah, pick up some igniters either from eBay or your local junkyard. I carry one or two in each car, even tho I've never in 22 years had one fail, but it does happen to people and it's perplexing.

B

 

Also check your earths, never hurts to add another one, and charging cable from the alternator.