Installed new V6 distributor. Cleaned rebuilt (but old) AC Delco mass airflow sensor. Checked timing with timing light. Disconnected electronic spark control for timing test. Timing mark still jumps all over the place. Seems to jump back and forth at least 20° variation. Runs pretty good on highway. Has lumpy idle.

Opinions on whether lumpy idle is a timing issue or a too-rich fuel/air due to something the ECM is processing?