Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
It sounds like you are probably on the right course with the idle valve on the throttle body malfunctioning. They are common failure points on these vehicles.
Before replacing it however I would first double check your belt timing is correct... if it has slipped a tooth it will cause it to have difficulty adjusting idle properly. Second I would carefully go over the entire engine looking for vacuum leaks. Not just checking for vacuum hoses off/rotted/etc, but looking for leaking intake gaskets, injector seals, etc. Unlit propane or brake cleaner sprayed at all potential leak points while the engine runs is the best way to do this.... if there is a leak it will draw in the excess fuel and affect the idle.
If the belt is timed right and there are definitely no vacuum leaks, then I would want to try another idle valve. While it is possible to have other issues (bad injector, fuel pressure problem, etc), it is unlikely to see things like that without a engine light coming on too. Surging idle with no light is usually going to be vacuum related or valve related.