'96 Taurus w/V6. Revving engine while in neutral does not result in "linear" increase in RPM, instead resulting in "hiccups." It'll rev a bit, then "hiccups" (combustion rate appears to slow dramatically for ~1-2 seconds) and RPM decreases significantly before heading up again. It's done this since purchase, and I'm unable to tell if performance while driving is affected (flooring it while in drive does not seem to replicate issue). Asked for diagnosis at nearby auto shop in middle of nowhere, received unauthorized tune-up for 1/4 the car's worth which did not resolve issue.
Is this actually an issue or is this normal behavior?
Excellent one to start off with! It's most certainly not normal behavior. Computer controlled vehicles, especially 1996+ (OBD-II), are very precisely engineered. Anything that seems "off" usually is.
Your problem, sadly, could have multiple reasons.
TPS(throttle position sensors) are potentiometers, and can develop a bit of a dead spot. While free-revving the engine in neutral, it dramatically exaggerates problems that might not be seen while in drive, because engine load can overshadow them. This can't be seen without an oscilloscope or very pricey scan tool.
Faulty fuel pressure regulating can also produce your symptoms, but I would expect to see issues while in drive as well. You can rent a pressure gauge from Autozone(fee refunded 100% on return of tool). Your pressure should be between 35-45psi with key on, engine off. It should hold steady for quite some time, too. Another quick test of the FPR(pressure regulator) would be to take the vacuum line off of it. If the diaphragm inside has ruptured, gas will be in the vacuum line(shouldn't be under normal conditions). When that happens, the vacuum of the intake manifold pulls fuel through the lines and into the cylinders without the computer knowing it, and causes stumbles and general rough running.
The tuneup that was performed might have been a step in the right direction. Ignition coils are generally "2-stage", meaning that the power they produce for idling is separate from the power they produce when throttle is applied. The secondary windings in the coil could be bad, causing your issue.
Lastly, the MAF(mass airflow sensor, big blockey sensor in the plastic part of your intake tube) could be the culprit as well. This is another one that will require a pricey scan tool to figure out. One thing you could do though is examine the filament inside of it. If you take the MAF out of the car and look inside, you'll see a tiny tiny wire going across the inside. This wire should be as white as a tissue. If it's not, you can clean it with some spray chemicals. Autozone sells MAF Cleaner. Don't use carburetor cleaner or brake cleaner as those are chlorinated chemicals and will destroy it.
As an aside, this particular vehicle has about 6 TSB's(tech bulletins) from Ford talking about rough running, bad idle, etc etc. They require a reprogram of the computer. You might want to give Ford a call(have your VIN handy) and see what they say.
All in all, given the age and (assumed) mileage, it might not be worth trying to find and fix the problem. If it's liveable and you experience no other issues, then I'd say.. keep on keepin on. Mine up some BTC and buy a new car! lol