Low Idle Out of the Blue

1985 VW Jetta Carrat 230,000 Klm. My car has run fine since we purchased it. I have had power issues where if I don't start the car after 2 days, the battery is dead. I replaced the battery and had altinator rebuild but still same problem. My question however is about the low idle. Out of the blue, when sitting at a light the car idles from 1000RPM down to 600 RPM then back up again and jerks and is noisy etc...The timing is also off. I have to turn car off in drive or press the gas while turning car off to prevent puttering. I have read other posts but not same issue, year or make as my car. Your help is appreciated. Thank You.