[syncroid]

Wow, Thats a good one. Heres a couple things you can try. Make sure your distributor advance is working. Pull off the vacuum line on the inboard side of the distributor. (idle should jump up if working properly) If not, your mechanical advance in the distributor is frozen or the vacuum advance pot is bad.
Its also possible you could have a severe intake vacuum leak. I have seen the big vacuum hose for the brake booster split and cause this problem.
Anyhooos.....just a couple of ideas to throw your way. Good luck!