Hi , The inlet manifold needs to come off so when i did mine i replaced knock sensors , starter motor , re-sealed the inlet manifold bottom cover and replaced the coolant hoses with the Roose silicone ones, all refitted with new seals. The idle speed @ around 1000rpm in drive appears correct, the hunting is most likely a vac leak or failed one way valve into the manifold, there are a few , again these can be checked/replaced when the manifold is off (there is a vac line + y piece underneath it) also check the brake booster kaiser valve , access will be easier when its apart. I recently changed the front two O2 sensors that made a noticable difference in the overall steadiness and smoothness of the engine so i would consider these if they've not been changed yet. Ive not had a problem reading error codes using a cheapy obd scanner, its a 2001 year so would have thought yours would be the same? This all sounds a bit drastic but a few of the parts are generic to other car makes ie starter, knock sensors, O2 sensors ,kaiser valve so it being a case of "while your in there you may as well".... good luck