1980 Toyota Supra Idle speed Fluctuations

I have 1980 Supra MK1 Non Turbo 5 Speed manual with about 69K on rebuilt engine. Car Just passed the smog test last week in CA. Car runs fine in all respect except its spped varies during idle from 800 to 1000RPM. I feel trhe same variations while driving at other speed and any of the gears.
There is another problem which may or may not be related. Once I drove about 100 miles and Car exhause manifold next to the engine got red hot.
I am not loosing any power and engine timing and spark plugs looks good.
Any help will be appreciated.
Thanks