I have a very similar problem. But still haven't found the cause.
My mechanic did the following and the stumble at idle still has not gone away.
-changed coolant temperature sensor, and verified proper resistance, good.
-changed fuel filter.
-checked fuel pressure, good (12psi).
-Rebuilt TBI with kit.
-new gaskets for TBI.
-new air filter.
-checked distributor bushing play, good.
-checked Idle Air Controller, good.
-checked operation of EGR valve, good.
-Installed new NGK plug wires.
-New spark plugs.
-checked engine vacuum, excellent.
My mechanic is quite stumped at the cause. He increased the idle up a bit and it still has a stumble but at least it doesn't nearly stall like it did before.
Hope you find the fix for this....