Off idle stumble

We are working on a 94 Buick Estate Wagon, 5.7L. The car runs excellent EXCEPT just off idle there is a hesitation/stumble. Once you get past that it has good power and exhibits no more symptoms or issues. We've noticed the spark advance drops to zero when the stumble occurs (from usually around 19-23 BTDC) but comes right back after the hesitation. We have sprayed some intake system cleaner through it, added injector cleaner (Sea Foam) to the tank and even tried a new TPS sensor, still the stumble persists.
The IAC has good values/feedback and the EGR system seems to be functioning properly. Any ideas/suggestions? The intake does have an excessive amount of carbon buildup.