I had that happen with a 1984 Rex...

When i got it the idle was set at around 2k, and when i tried to set it to idle around 700, a similar trouble had happened. It idled fine around 7, but once it warmed up it jumped up to 1500 and back down to almost shutting off and then back up again, no one knew wtf, so i just put the idle back up until it didn't "jump". My girl totalled the car in the snow before i could really try to fix it...