If you vehicle is due for plugs and wires do it now, that may take care of
your problem

Hey Janne,

 What you have described so far isn't possible.  Here's what I'm hearing
you say:

1) Backfiring at part throttle, not full throttle.

2) Ignition system completely checked out with a friend's scope and timing
is correct and all parts of ignition system have been replaced

3) Car completely within emissions.

In short, this isn't possible.

So, let's go back to the beginning and start over.

If this was a spark plug/secondary ignition problem (bad coil, crossed
wires, etc.) then
the misfiring/backfiring would get worse the higher the rpm.

If this was a spark primary ignition problem (ie: timing) then the friend
and his scope would have caught it.

If this was a restricted fuel filter or bad fuel pump then the higher the
rpm the
worse the problem.

If this was a compression problem it wouldn't be backfiring.

If this was a fuel problem then the emissions would be out of wack.

So I have to conclude that your testing methodology is flawed.  If you are
absolutely
positively positive that the timing is perfect and the ignition system is
perfect - which means your friend and his scope tested it out correctly -
then the problem is in the fuel system,
specifically the engine is running too lean - which means that the HC would
be high and
the CO way low - which means the emissions would be off.

In short, you cannot have it both ways.  You cannot have a perfect emissions
report
AND a perfect checkout with your friend and his scope, and still have this
problem.
One of them has to be wrong.

Off the cuff the most likely candidate is bad timing at part throttle, OR
too lean
condition at part throttle.  However both the timing and the mixture are
computer
controlled.  There is an old saying with computers, garbage-in, garbage-out.
I think
the likeliest problem here is you have one or more bad sensors feeding into
the computer.

The fact as you say that sometimes it runs fine is even more an indication
that it's
a sensor-to-computer problem.  Maybe a bad sensor that with the vibration
occasionally "heals"

Some would say that if it's a bad sensor that the computer would set a code.
Well I
have a '95 T&C with a 3.8 and when I bought it 3 years ago used, the EGR
valve was
shot, and the van wouldn't pass emissions, and there was NO CODE set.
PERIOD.
Replacing the EGR valve dropped the NoX down and it passed emissions.  So
don't
argue with me and tell me that the computer in these things is smart enough
to detect
if a sensor is shot.  I know from experience this is just wishful thinking.
Sometimes
sensors will fail in such a way that the computer cannot figure out that
they are bad.
For example, your TPS could have worn out the middle of it's slide and the
computer
not know where the throttle is, fuel mix is going to be wrong, then.  Car
computers
are notorious for saying one part is bad when it's another part entirely.

You say you have already shotgun the ignition system components.  Well fine,
great,
quite replacing them.  Or better yet keep replacing them and send me your
old ones
because they aren't broken.

It's time you put a scan tool on this vehicle, or pay someone to do it.  If
you really want
to do it yourself, you can buy an OTC Monitor 4000E off Ebay pretty cheap
that will do this.  You need to scan it and run the engine and make sure
your actually getting
valid inputs from all the sensors.  You need to check timing with a timing
light - yes I
know the factory manual says timing is non-adjustable, but a timing light
and degree wheel on the crank still work.  The cam chain could have jumped a
tooth and that is going to
shift timing out of wack.  You need to put a fuel pressure guage on the fuel
rail and test that.  In short, you are past the stage of being able to fix
it with a $39.99 on-sale Sears Craftsman wrench set, a Haynes manual, and a
bucket of miscellaneous screwdrivers.  You are either going to have to go
out and buy the tools (and documentation, like the Factory Service Manaul)
to troubleshoot it properly or pay someone who has the tools to do it
correctly.

Ted