This is most likely an ignition problem.  I'm guessing that it happens under
light/moderate throttle, right?  If you give it enough to gear down or
unlock the converter, it goes away, right?

I don't remember if you have COP or not, but I doubt it.  If you have plug
wires, I'd suspect the wires first.  If not, plugs are most likely.

There is an issue on some Ford V-8's of misfiring due to bleedover between
plugwires that are misrouted.  I forget which two, but there are two
adjacent cylinders that fire sequentially and often the wires for them are
run side by side.  If that's the case, there is some bleedover between the
two wires resulting in misfiring and weak spark.  I may be thinking about
the 5.0 tho...  I'm just an amateur who has a decent amount of Ford
experience.  Someone else will be able confirm or correct that.

It's theoretically possible that you have a TPS issue, but since the problem
seems more related to speed than throttle position, I'd suspect ignition
first.

CJB

Ok the 5.8L V8 is bogus if its a 98. Ford doesnt list the 5.8 in 98 F-150
PC/ED (thats the Powertrain Control/Engine Diagnostics manual). So I would
say you have a 5.4L. In that case, I would be willing to bet you have
problem with COP going bad on a cylinder or two. It is possible it could be
a PCM issue as that controls which COP is firing at that time. It could be a
bad crank sensor, as that is what the PCM uses to keep time. Could be a bad
TPS sensor and it could be "skipping".  The list here is ENDLESS. Does it
only happen in a certain gear? Can you hit the "OD off" switch and make it
stop?

Your best bet is to have someone run a stress test on the COP system, that
is depending on which engine you have. A stress test will tell if you have a
bad coil. I suggest you raise the hood and look at the EPA sticker under the
hood. If you see 5.4 or 5.8 anywhere that is the engine size. Its usually
listed in the "Engine Family" area on the sticker. Then check back here to
verify the engine size.

The best thing to do at this point, is that I know that WDS or IDS has the
ability to watch TPS, cylinder firing and watch sensors as you are driving.
I would take it to the dealer and have them hook it up and take it for a
test drive. That way they can see exactly what is going on at the time of
the problem.

Check back and let us know.

I had a similar problem right after the transmission was rebuilt.  I
changed everything related to the engine but the problem did not go
away.  I strongly suspected that there was a some kind of
slippage/grabbing somewhere in the torque converter or band somewhere.
The problem only occurred when climbing a grade.  I live in the
mountains.  I just lived with it as nothing could be proved one way or
the other.  As time when on it seemed to be less of a problem.