rb26powered

First problem I've encountered since owning the car.
84 GSL SE 13B
-When the car is warmed up and just sitting idle, it starts to idle erratically like a lazy bridgeport lope. It's a controlled erratic idle, same pattern up to about 1500RPM in about 1/2 second intervals for about 10-15 seconds then it idles normally.
It happens about once every other day.
Any insight?

Separate issue, I noticed I had a slight exhaust leake from the exhaust manifold to the main pipe flange, the color looked almost a light cream color, I thought exhaust leaks should be black? Is it possible that the fuel mixture is too lean?
Haven't checked the plugs yet.

thanks to anyone who can give me a clue.