78 911 3.0 started running very rough any help??

[blackhawk]

car is a 1978 911 with a 3.0. no A/C, smog pump removed for now.
the car just started running bad. not sure if it is related to the cold but one night i was driving and it started coughing (backfiring while driving when i would step on gas) then when i pulled off highway onto surface streets it ran very rough, when at idle the RPM's started going up and down from 950 down to about 700 then back up and so on. also i noticed the tach was jumping a bit as i was driving, so i would be at 3,000 rpms then it would start to jump to 3200 or 3300 then back down it did this for a while. just fast jumps. also with a def power loss as well.
one other note, the next day or day after it drove fine but then it went back to running rough. even on a much warmer day. on a cold start it starts right away but the idle is rough hunts up and down and small backfires when i rev it. i have replaced plugs not too long ago, replaced fuel filter, i have a K&N air filter and a MSD ignition with fairly new wires. any thoughts, suggestions or advice would be great. thanks

[makaio]

Have you checked the cap and rotor?

[timmy2]

Vacuum leak somewhere? Crappy gas?

[Targalid]

If the tach is bouncing without the engine changing speed while driving you have an electrical problem since the tach is driven electrically. Search along the connectors for the ignition system and see if you can find the problem. Might just be a loose connector or it could be the voltage regulator which will give a bouncing tach. Check the voltage of your charging system.

[blackhawk]

thanks, i am suspecting vacuum but the electrical weirdness also makes me think although it may be both since the tach issues don't always seem to happen when it is running bad.