You would have gotten quicker help by posting this in the Tech forum. I have a very complete listing of trouble codes tailored specifically for 5.0 Mustangs, but I seldom check the tuner forum.
Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code may prevent you from running the Key On Engine On tests. You can generally ignore this code, since it has no effect on engine performance.
The computer wants to make sure the A/C is off due to the added load on the engine for the engine running tests. It also checks to see that the transmission is in Neutral and the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the car from being driven while the computer is in test mode. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis.
i have run through almost all of the "surging idle" checklist
when i dc the iac i can set it to about 600 rpm but it does surge by about 100 rpm
i haven't replaced any vac but it all looks good
i have replaced or tried w/ a good one of the following
plugs(gapped .040")
wires
coil
msd
cap
rotor
maf(both the body and sensor separately)
tps
iac
timing @ 12*
i have reset the puter after any changes
still hunting a low rpm stumble, it smells unusually gassy (i have owned a 5.0 w/ LT's and offroad b4 so i am used to what that one smells like)
ET seems off by 3-5 tenths, mph is off by about 3 mph too
the miss is at idle to about 2000, i don't feel it after that, but feels like a stutter, kind of shaky and worse when i'm driving it rather than just giving some throttle
i have a buddies distributor i can swap out to try, but after that i'm lost
i feel i have eliminated 98% of the possible prob, and getting irritated