timing that's a little off may cause pinging, but not usually a stumble or "invisible wall". From what you're saying I'd say it's your PIP sensor. People are probably getting tired of hearing that, but let's face it, these cars are getting pretty old now and even those with low mileage cars are starting to see the effects of their cars aging.
I had those symptoms, 10* timing-NEW BALANCER, 93 octane, still pinging and stumbling/hesitation. PIP/new dizzy fixed those problems.
+1 on the new dizzy. I had similar issues around 68,000miles but once replaced she ran much better. FYI: I did the whole ignition system in one step-Mallory box, coil, wires, plugs and dizzy. Money well spent.
its been said before and I'm going to agree. plugs, wires, cap and rotor, maf clean, air filter (who knows last time that was done), and fuel filter..... doing this took care of the same problem, had it last summer. well minus the fuel filter that was done earlier.