Try spraying a little carb cleaner through the little red tube at the base, at the throttle shaft, along the intake manifold where it bolts to the heads and any other possible location where a vacuum exists and could leak first. If you have the engine equipped with an EGR valve, check to make sure it is closing completely. If you had to add a carbon canister (CA AQB requirement in some instances) try bypassing it.
Also check fuel pump output for pressure and volume.
If the carb flange or spacer are leaking, take a look at the gasket surfaces to make sure they are clean, and then grease both sides of the gaskets to be sure they seal (do not use regular sealers- you may want to get the carb off in the future) and check the sealing surfaces to make sure they are flat (.003 inch of distortion across the plate or the base is the maximum allowable with thin gaskets .004 with thick). Pay attention to how the bolts or stud nuts are getting tightened so that the whole assembly is tightened evenly.
If that turns up nothing, check the distributer to make sure that the wires to the pick up coil are in good shape (if it was used, the wires may be broken internally and make intermittent contact.) also check to see if there may be an issue with the distributer gear climbing up the cam and then dropping to run in a different spot (bent distributor shaft and or too much end play in distributor shaft)- Check with a dial indicator
The next step is to to look for cracks (or possibly even the wrong airhorn gasket )in the float bowl that might allow gas to leak uncontrolled into the low speed circuit or into the throttle bores, especially the secondaries. Make sure the metering rods do not get hung up anywhere either.
Also if you are running vacuum accessories, make sure none of them have an air leak or broken hoses- including the line for the modulator if you are running an automatic. Remove and plug the port for each one one at a time if need be to test. Also check brake booster for a potential source of a vacuum leak too.
lastly, if you have a high lift performance cam, the engine may only be able to idle at 1200RPM, so check the cam manufacturer specs if anything but a stock grind cam. And if you are running a single plane manifold as opposed to the dual plane manifold (look like stock manifold when looking through the 4 bore holes in the manifold) you are not going to get a stable idle below 1200 RPM.