What everyone else said. A jumpy tach is usually a sign of ignition problems. The reason you're not getting much in the way of other solutions is that we've already been there and seen it a bunch of times. Unless you just replaced all the "consumable" ignition parts within the last week, that's probably where your problem is. Many times bad parts "look" ok. These trucks are really picky about ignition. If anything looks less than perfect, you should probably change it.
I generally replace spark more often than the other parts, due to being right there in the combustion chamber, which I figure is a bit hard on them. But if I'm having problems, I replace the cap & rotor, wires, and plugs all at once. You'll usually end up changing all of them anyway while you're trying to track the problem down, so it just saves time to do it all at once. If you still have problems, it is possible to get defective parts. I'd try another set just to make sure, before deciding the problem is your uber-expensive flux capacitor or something. Also, make sure everything is done right. Those caps have been known to go on crooked and other little stuff like that.
Get a scantool if you don't have one. That's the only way to know for sure what the chip is telling your truck to do... I don't think it'll kill RPMs like that, but if you have some other problem, the ECM can make the engine run funny in an attempt to avoid damage.
My S-10's fuel filter got so clogged it wouldn't run. It'll act the same as if you're running out of gas. In effect, the same thing is happening; the engine just isn't getting fuel. It'll spit and sputter, espcially under lots of throttle (when it wants more gas but can't get it). It probably isn't your fuel filter, but you should probably change it just to be sure. It's a relatively painless procedure, unless you've just gouged your hands up changing ignition stuff (gasoline + open wounds = OUCH!).
