I would start at the tb and check it all for vac leaks. The hose in ear method will find all but the smallest if you can probe everywhere, which is not easy. There is the tb/water plate joint, the water plate/plenum joint that are gasketed, the isc oring, the various vac lines, the plenum casting itself, the upper/lower plenum joint, and the lower plenum/heads pair of joints.

One way to check for vac leaks in individual intake runners (between the plenum and the intake valve) is a vac gauge plumbed to idle vac, not ported vac. Watch the gauge at idle. If it ticks down once every two engine revs, you have a leaking runner or some other single cylinder sealing problem. A vac gauge is a dandy and very cheap diagnostic tool. Google using one for an education.

The wax pellet fidle valve is also a suspect, despite what I said earlier. If it failed open, you have a good sized air path right there, and the isc can't cope.