'98 Windstar Idle 'Hiccup'

[ibia22]

Hi,
My '98 Windstar, w/125k miles, hiccups or stutters at idle every 2-3 seconds. The symptom is further characterized by a low frequency thud or popping noise and an ever so slight 20 or 30 rpm drop/glitch of the tachometer needle, that otherwise sits pretty at around 500 rpm. And it happens in or out of gear and with or without A/C compressor cycling.

Some things done to Windstar over the past 6 months:

- New Auto Lite double plat plugs
- New DPFE sensor and EGR valve
- New Left and Right IMRC motors
- New battery
- New air filter, fuel filter, and PCV valve
- Cleaned Mass Air Flow sensor with no-residue electronic comp cleaner
- Cleaned upper intake manifold with B-12 Chem-Tool (spray and pour can)
- B-12 Chem-Tool gas tank treatment - 3 times
- New Vapor Canister Purge Valve
- New Idle Air Control valve

No CEL. And motor runs great and pulls strong at full throttle. And consistently gets over 300 mi. per full tank of 87 octane gas split between city/hgwy driving. What could it be?