Hard stumble under load

**Rittercon** · 04-17-2019

I have an intermittent "hard stumble" in my '94 Chevy 350 pickup. It occurs when the truck is in the 1,700-ish rpm range and MAP is at 75 KPA-ish. It's a quick event; the motor takes a hard stumble and then recovers. It doesn't happen all the time at that rpm/map range; just sometimes.

I can see the event in my TunerPro log files, but don't know what's causing it. Here is a table that shows the event, plus a few before/after consecutive events. I've highlighted the event in bold italic and used red font for categories where obvious differences occurred, some of which may not be relevant.

TunerPro Sample #	Battery Voltage	Block Learn Add	Block Learn Cell	BLM	Fuel Pump Voltage	Fuel Usage	IAC Desired	IAC Preset Motor Position	Inj DC%	Inj BPW	MAP	Spark Advance	ECU Loop Ref Period	Tach RPM	Knock Counts
6765	13.4	FALSE	10	125	13.7	14.7	68	68	10.48	3.75	74.2	29.9	1666.17	1675	303
6766	13.5	FALSE	10	124	13.7	14.9	68	68	10.35	3.71	73.8	29.9	1666.17	1675	303
6767	13.5	FALSE	10	124	13.7	15.1	68	68	10.27	3.68	74.2	29.5	1666.17	1675	303
*6768*	*13.4*	*FALSE*	11	*124*	*13.8*	*3.9*	68	68	*40.38*	*3.85*	*74.2*	*19.7*	*6301.54*	*6300*	*303*
6769	13.4	TRUE	10	124	13.8	14.4	105	77	10.73	3.85	74.6	23.9	1683.29	1675	303
6770	13.4	FALSE	10	124	13.8	14.6	105	91	10.73	3.85	76.0	26.0	1666.17	1675	303
6771	13.4	FALSE	10	124	13.7	14.5	105	105	10.82	3.88	76.4	27.1	1666.17	1675	303

The knock counts start at 303 and don't change the entire log and there's no knock retard recorded.
I'm running a BHMX 16196395 $0D chip that I've modified to improve the VE and Spark curves. The truck has a mild cam, Summit aluminum heads, shorty headers, stock TBI & manifold, all CA emission approved.

Greatly appreciate any thoughts!