I had same problem
see my post "misfire at cruising speed"
after hunting down the problem and exhausting every effort i decided to pull the distributor (MSD digital E curve) and install my old standby mallory unilite.
upon pulling the dist I noticed the gear kinda looked funny.
seems like it is wearing oddly.
I removed the gear and drove to the local speed shop and got a replacement bronze gear.
Keep in mind now that my dist had less than 300 miles on it.
anyhow I replaced the gear and reinstalled dist.
problem went away!

I had a similar problem to this once and it ended up being the tach shorting internally to ground. :?

Also your cam duration can play havoc to the timing if your not using the correct timing curve for your cam'
cams with less than 220 duration @ .050 =10 to 12 degrees of initial timing.
cams with less than 240 duration @ .050 =14 to 16 degrees of initial timing
cams with less than 260 duration @ .050 =18 to 20 degress of initail timing

Try making a run with the tach unhooked. I had a tach short out among several other guys. Try that. That will mess with the ign. system and cause stupid plug readings also.

what is a floating ground?

floating ground:
when common connections providing a return grounding path exist, but are not actually connected to an earth ground

latest results
 

Just got back from the hottest day of the year here in ATL.

What I have done since last post:

Installed ground strap from block to chassis
new ground staps fine strand from battery to chassis )in rear)
slightly corroded at contact, but nottoo bad
shielded trigger cable
msd noise reducer
checked wires for ohms good
cleaned plugs, all look the same running slight rich

from before soldered all connections & connectors
hot & ground msd to battery
r&r cap,rotor,mag pickup, wires

Ok yesterday 1st run same thing
2nd run re routed coil wires
same thing

On Barry grants site it said cowl induction hoods without AirCleaner can cause this type of issue

run 3 took the hood off, put on Air cleaner , summit 14 inch cheepie, moved alternator wire & mag pickup cable(i know i should only do 1 thing at a time, but this was rnd1 & NOT A t&t

picked up.2 only did it in 2nd gear instead of both gears
I was bedding in new brakes so I used the last turn off & hopped on it a few times & noticed the fuel pressure guage hopping around(full tank BTW before run)

As fluke, I loosened the fuel cell cap ,

run 4, I dropped my dial hoping for the best, But not enough, ran .2 faster on the brakes, best 60 ft since I owned it, no break up, no fuel pressure guage fluctuation (that I noticed)
Unfortunately I broke out so my day was done, so I cant repeat it,

BTW when I pulled off the air cleaner it was stained brown/black discolor under lid.



Also I want to thank everyone for their input, my backround is fuel injected road race cars & I am learning a lot.

Re: latest results
 

[quote=

As fluke, I loosened the fuel cell cap ,

run 4, I dropped my dial hoping for the best, But not enough, ran .2 faster on the brakes, best 60 ft since I owned it, no break up, no fuel pressure guafe fluctuation (that i noticed)
Unfortunately I broke out so my day was done, so I cant repeat it,

.[/quote]
I think I would look real hard at that fuel cell vent, if the pump is pulling just a little vacuum,on the fuel cell, it could cause these symptoms. you said in your first post, I think it was, that it sounded like a lean miss, maybe it was. It sounds like you have covered the bases on the electrcal end. And I believe you said the fuel cell cap was loose after your dyno pulls. Just a thought.

Yeah,I agree with suicidebomb.

engine miss
 

No one on this thread has mentioned temperature where the MSD unit is installed.I am no expert by any stretch of the imagination but like all you door car people installed an MSD on the passengers side floorboard of one of my cars awhile back.The car developed a miss going through the traps.I spent weeks trying to get to the bottom of that problem rewired the entire car new MSD,coil,plug wires,crank trigger acouple thousand dollars later I was at my local track with the car it was on a very cool fall evening had no problems car ran fantastic.I never had the problem the rest of the season.Built another motor over the winter and didn't get back to the track until late May hot day. 1st pass on motor engine missing going through traps WOWwas I frustrated. Think About it the header was right under the MSD I took a household oven thermometer to the track with me.That floorboard was 171*f going through the traps and that was my problem. Something I learned through all this is electronics and high temperature don't mix well.