First off, I don't know this specific engine.

In general, I would try to look at ignition advance at part throttle.
Does the distributor have a vacuum can on the side? That would normally
give you an initial kick in the advance just as you begin pushing on the
gas pedal, so the engine would accelerate from idle. It also allows
higher ignition advance at cruising speeds, with small throttle
openings. If the diaphragm inside the can is leaking, or if the hose
connected to it has a leak somewhere, it won't give you that extra
advance and the engine won't respond well to throttle.

Also make sure all moving parts inside the distributor body are clean
and lubricated, and move well without binding. This goes for the
centrifugal weight mechanism parts too. If those are binding, the engine
won't see enough ignition advance.

To verify either the vacuum advance or the centrifugal advance
operation, use a strobe light timing gun and observe the timing changes
when the engine is running. Repeated very small throttle openings should
result in instant ignition jumps (vacuum advance).
The centrifugal advance operation can be checked with the vacuum hose
disconnected and plugged. If you get no change in timing regardless of
engine rpm, then the centrifugal mechanism is not working.

You can also try to suck on the vacuum can hose to try to detect leaks.
You should be able to suck a vacuum in the hose, and block the hose
opening with your tongue... it should stay stuck to the hose, meaning
the vac unit is able to maintain vacuum with zero leaks.
You can also buy an inexpensive manual vac pump from most auto parts
stores, if you don't like getting your mouth dirty :)

Another problem may be incorrect mixture. Troubleshooting and fixing
this may get a bit more complicated and vehicle specific, so I won't get
into that here.

Jan

Like Jan, I don't know the 4 cyl engine that well.  Everytime I fill
up, I thiink your friend has the right idea - he / she gets the
Mustang experience with a little less bite at the pump. :-)
Anyway, on the 8 cylinder, I'd look at the idle air control, if
present on that motor, and possibly the TPS.  I heard both suspected
in low idle driveability concerns, although Jan's vacuum ideas are at
least as good.  The IAC gets gummed up over time, and is an expensive
piece to replace, but 20 mins with a can of carb / throttle cleaner
has NEVER failed me.

I am going to help my friend tomorrow, we have two different service
manuals and hope to check out sensors and find one that may be at
fault.  The spark advance is controlled by a TFI electronic control (I
replied initially to this posting that fact, but it never showed up),
and I've read that it can have problems with heat.  I plan to try
quick cool, etc to isolate.  What my friend has told me is that the
car does run fine for about 5 minutes when cold.  Fine, but why would
problems with TFI getting hot, be apparent only at low RPM, what would
be the symptoms be for a heat failing TFI - no spark advance at all-?
How would it work fine at higher RPM and not low RPM if TFI is
failing?  It would seem to me that something is giving the computer
control module band info at for low RPM and I will be checking the TPS
for one.  Any other suggestions, opinions is appreciated.

Thanks