I don't know what would cause this, seems like an ignition thing since the tach drops with it. Like the coil isn't even firing at all for a stroke. You can hear the engine faintly pop when the tach drops. More towards the end of the video.
http://gtluke.smugmug.com/gallery/5470093_YkMGr#620137339_mSsBo-A-LB

Here's another really short one where it gets pretty heavy.
http://gtluke.smugmug.com/gallery/5470093_YkMGr#620127899_ZQ9Pv-A-LB

It only sometimes does it while accelerating. It really comes up the worst when cruising but I did get on it a few times today and once or twice it was stumbling like that while accelerating.

Car has an AEM EMS, stock ford dizzy, painless wiring harness, 351w with hydro roller cam, oldish stock replacement accell coil.
I just pulled the plugs and they are all gapped fine and are in good shape. Cap looks fine, pretty new.
Engine is on solid mounts so it's not an engine ground problem.

 

I had a similar problem once . Mine ended up being a frayed tach wire touch metal behind the dash . It was crazy looking for that . It took taking every ignition related wire out of the car . I actually thought it was boost related because that was the only time it did it . But it was the wire touching metal every time I got on it , the tach would jump and cause a mis . Driving around normal it never touched . I have no idea why a tach wire would cause a miss when shorted like that , but it did . It was an Accel 300 box .

Anyway , sorry 'bout my life story . lol

Good luck .

 

My guess is it's the AEM. They are famous for "timing errors". If it is timing errors ytou may/may not ne able to get rid of it easy. I ran the AEM a few years back got got rid of it for this reason. You can try playing with the dwell factor and MX setting. What do you have for these settings now?

Also make sure your dist is in good condition.

 

Thanks guys. My wiring harness is actually a brand new painless harness, hopefully it's not wiring. That would surely piss me off.
I was thinking it was the AEM, I have the factory ignition settings from AEM, whatever came with my unit like 4 years ago when I bought it. I guess I should see if there is a firmware update or mustang ignition update.
What am I checking with the distributor? horizontal play? It's got a newish pickup coil in it, the one that worked forever in my car wouldn't give a signal to the AEM but the new one did. hmf.

 

Is it spark scatter ?? I had issues like this.. Finally went dis system, wasted spark.
No matter how we phased the rotor..
Yea playing with leading edge etc on ECU pick up settings ??...

 

Check that new-ish pickup. I put a new pickup in my race car at the Montgomery ORSCA race and it started missing on sunday. This saturday I finally found it.. The pickup had made it's way into the reluctor. I'm amazed it ran as well as it did!!

Just one more thing to check....