RE: High speed sputter
I was going to suggest something like spark problems... I was actually going to suggest you look into how your knock sensor is doing. That sputter may have fired of a DTC light that has shown your KNOCK sensor or should have shown in the sensor. Knock sensors control timing advance of your vehicle. If you don't know, the timing is what controls how your plugs fire due to fuel and air input into your engine. If they claim bad plugs and wires (it's rare to have bad wires), I'd suggest you look at the output of your knock sensor.
You can perform a self test of this, sort of. First, listen to your motor while driving, and see if you hear what sounds like a small bag of marbles rolling around inside your engine block, it could be very subtle. You won't hear this much at an idle, or without driving it, so reving the engine most likely won't produce these sounds. Acceleration while under load, or at higher RPM's, or even firmly accelerating up the street from a low RPM to a higher RPM. Depending on the year of your vehicle, it may not even have a knock sensor. Sometimes this sound isn't even auditable to the naked ear. These sensors are too expensive either, it's dependant on where in/on your motor the sensor is located, typically 30 to 50 dollars, however labor to get to it may cost some.
Also, if your plugs were bad, think about the possibility that your injectors are starting to be effected, if you don't have feul injection on your motor, then pop off the top to your carburator, and see if you have carbon build up on the air chokes, or walls of the carburator.
This is just a "things .look for" suggestions, as it's highly possible that new plugs and wires was the trouble. I just don't want you to be too comfortable about your problems being fixed, as something had to cause your plug and wire problem. Bad gas???? Bad alternator to burn out the wires? Bad distributor, or alternator? I'm not an expert, but that's my experience.
Hope all goes well 4 u!
2003 F-250 SD 4x4 Auto, Ext Cab SB
SOHC 6.8L V10 Factory SAE 315HP
GIBSON Dual Exhaust & Perf Headers +15HP
K&N Cool Air Intake + 10HP
BANKS TransCommand PCU +12HP
HypterTech III Perf Prog +18HP
Dyno: 368HP@4518/Torque 538lb@3400.