so it starts fine but can't hold an idle. it runs completely fine higher than 1500 rpm? what exactly happens when it "rumbles"? is it just at a really low engine speed like it's about to stall? if you hit the throttle just a liiiittle bit, is it ok?
if it doesn't die and just kind of idles rough, perhaps it is a clogged injector or fuel filter or something. maybe you could try running some of that injector cleaner in your next tank of gas.
i'm not really familiar with these things, but maybe it is a bad idle speed control valve or maybe even a tps with a bad idle switch. the idle speed control valve is controlled by the computer to control the air mixture at idle. the tps has a switch for when the throttle's closed to tell the computer you're at idle. i'm not exactly sure of easy ways to go about testing the two, but i'm guessing it would make sense to check the tps' switch first. if enough of the tps harness is exposed and you had a voltmeter with nice and pointy leads you could hopefully easily test it-- hook one lead to the blue wire, another to the brown wire, and see if it changes with teh throttle completely closed versus it being a little open.
i think it should be ~5v when the throttle's closed.
if the switch is working... i don't really know how to test the valve. i think it's a stepper motor, if you find power and ground leads. if you can get your hands on a haynes manual i bet they'll describe a good way of testing it.