The beast likely has a mechanical throttle (cable) that passes over the cylinder head and beneath the airbox, like the Cinquecento.
First of all, get your head into the engine bay, find the throttle butterfly lever on the throttle and move it to it's fully open position. Then let it go. It should snap immediately shut.
If there's any sluggishness caused by a sticking cable (the lever will be moving to close the throttle but the cable is bulging out), then your cuplrit is the cable. If the airbox is removed and then put back on with the cable beneath it, it can trap it against the head. The head moves during use, and can fret against the sheath of the cable (it's a regular Bowden cable, like a handbrake cable). Then if the inner metal winding gets deformed or corroded, that will affect the inner cable's ability to move freely. The cable should pass next to, not under the airbox.
If the cable and throttle are closing smartly but the beast is still being an arse, then it could be a dodgy sensor somewhere. The main one is likely to be the MAF sensor which I think sits in the top of the metal throttle body on the SPI at least. You need to get a magnifying glass and have a good look at it. It should look similar to a light bulb filament only a little square "microchip" thing, supported on tiny wires.
Clean it using a brake cleaner type spray. If it looks gone AWOL or bent, then it could be knackered and no amount of cleaning will save it. Also there's maybe/must be am electrical plug to the throttle body, to power and read the sensor. Make sure not loose or dirty. I don't know what readings it ought to give on a voltmeter but if it's clean and a good fit, then it's good enough for now.
Check the beast has no air-leaks on the inlet (from where the airbox seals against the throttle body).
Remove the spark plugs after a longish drive and check what they looks like. If one is much cleaner than the others, then there's possibly an engine fault (head gasket lets in coolant which washes the plug). You'll be losing coolant if this is happening. The engine "hunts" because it's running rich... it's getting fuel but instead of air, it's sucking in coolant.
If you have one lying about, also have a look at swapping out the lambda sensor. If you can get an emissions test done more cheaply than just buying a new lambda and putting it in there, that will show you whether the emissions are good or not. Knackered/way out emissions could be a lamda (or a faulty cat' .. but a knackered cat' doesn't usually affect the idle so much).
If you fitted a cone filter... put the original airbox back on. Possibly marvel at how all the problems go away..
Anyways, just be methodical and work your way through the most likely stuff first. Don't ignore any strange symptoms but also don't get distracted by stuff it can't be, like engine timing or TDC sensors etc.
Ralf S.
