I have a 93 mustang 5.0.. I've been having some weird problems that
no one has been able to diagnose... I'm at my wits end trying to fix
it. First off here's the symptoms...

It starts up fine, it idles ok, but with an occasional miss. For the
first few minutes if u give it hardly any throttle at all.. it will
backfire occasionally or will buck if you try to accellerate too fast..
accellerating slowly, getting up to speed and then flooring it results
in some burbling misfiring 0 accelleration, it coasts down until you
go back down to about 1/8 throttle. Once the oxygen sensors heat up -
I have an a/f gauge, so when it starts bouncing around - it will drive
fine up to about 2/3 throttle.. at cruise and idle it is fine, can not
tell anything is wrong. But once close to full throttle it bucks and
will backfire occasionally.

I pulled the plugs.. every one looks good except for cylinder #5. It
has quite a bit of deposits on it from something.. it's not dark, it's
kind of a grayish green ash. So seeing this I'm thinking head gasket??
It does use a little bit of coolant, refill every few months. It
doesn't smoke at all, there are no bubbles in the radiator, and I
haven't had a chance to do a compression check yet. I have replaced
the ignition coil.. plug wires, plugs.. the o2 sensors were replaced
about 12,000 miles ago, i'm considering replacing them again.. I had a
couple plug wires burn through about a year ago, possibly contaminated
the sensor on the one side.. but that doesn't explain the other side.

Pulling codes from the computer, it gives 3.. a tfi/spout connector,
and both banks running lean or fuel pressure low. I checked the spout
connector today.. the side going to the tfi gives about 5 ohms, and the
side going to computer gives about 10 ohms. So it's not shorted.. I
didn't check voltage while it was runnning, was planning on replacing
the ignition module tomorrow anyway. I can take the vacuum line off
the fuel pressure regulator and it does not drive any differently, so
probably i'm looking at a fuel flow problem, fuel pressure regulators
are cheap so maybe I'll try that too.. I don't have a fuel pressure
gauge, but I can get one when i get an ignition module.
So tomorrow or wednesday after school i'm going to replace the fuel
filter and do a pressure test, replace the ignition module and possibly
do a compression test. About the only other thing I can think of is
the cat converters are clogged, or the computer is messed up which
would explain why it runs bad when the computer is controlling the a/f.
Any suggestions would be appreciated.. Thanks!