Engine miss

I have an 86 D21 pickup with a 6 cylinder. At about 1800 to 2500 rpm's the engine rpms drop to close to idle speed for a second and just as quickly return back up. It will do it over and over again. It does it when in neutral or while driving. It provides a constant jerking motion while in that rpm range while I'm driving. If I go beyond that rpm range it or below it it runs perfectly. The Nissan dealer gave up and suggested I try replacing the throttle body; which I did (used one). I've also done: Complete tune up (plugs, wires, distributor cap, rotor, fuel filter, pcv valve and vacuum hoses). I've replace the complete AIV assembly (though I don't think this has anything to do with it). The dealer put a new oxygen sensor in. They said the distributor (optical sensor) was okay. I cleaned the (under seat) computer connections. Other than this it runs great but nobody seems to be able to figure this one out. Thanks for any advice.