Tach jumps all over the place driving and idle. works correct sometimes. how to repair?

AIC (Air Idle Control) valve. They will get full of carbon and the plunger will not move freely. Sometimes they can be cleaned and work properly once again, but usually they need replaced,

You need to have the tach replaced or take it out and have an auto repair shop or the dealer send it out for repairs, that is if it's repairable.

Tach spring is breaking, you can get a seperate tach gauge in place of what you got. or how about this take dash out and get another tach gauge they are removable

THis is a problem with your VCC gear in your transmission.

tach not working

possible ignition coil itself because tachometer takes signal from ignition coil.you may also have check engine light on too.

First, have the battery charged and load tested. If it does not pass load test, replace it. An alternator is not made to charge a battery. it's job is to maintain the current state of charge. By driving and attempting to charge it that way you over-worked the alternator and may have damaged it or the regulator. Once the battery is fully charged and in good condition, you will need to check the charging system for proper operation. If the gauge system does not return to normal, it will need further diagnosis to correct it.This isn't a "cure" but rather, what you need to do in order to get back to "square one". With the alternator running "flat out" trying to charge a dead battery it can put out over 15volts, possibly damaging the dash/body control module or computer.

you most likely need a new alternator

have you checked the EGR (exhaist gas recirculation) valve? sometimes carbon will chip off and make them stick partially open causing extremely rough idle or wont idle at all without help from throttle

maybe the speedo drive has stripped some teeth. good luck!