I'd guess that either clutch packs or gears are bad or the controller is giving bad commands OR you have a vacuum leak. I can't remember which years had the vacuum lines for the tranny, but you might have one of those years.
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WizAire CAI w/9" K&N & HV3, 2.5" d/p w/h/f cat, Cobra CB, PowerSlot slots w/Hawk HPS, GR-2s, GMPP handling kit, NextLevel STBs, Pilot Sport A/S Plus tires.
98 L36/MM5
RaC