Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
I think you are probably on the right track with a vacuum leak suspicion. What I would be doing here would be using brake cleaner or propane to check for leaks while the engine is idling. If you direct the added "fuel" around the intake, etc, and you get near a leak, it will draw in the extra fuel and cause the idle to change behavior.
Otherwise my next top three suspects would be:
-Idle speed control valve (not an issue if you changed out a new throttle body as it should have come with this), as they can cause irregular behavior at idle if they stick.
-Timing belt being a tooth off, particularly on one cam only for example
-Fuel pressure issue (not necessarily a pump.... usually we see the regulators go bad... but a pressure issue of some sort anyway).
Unless you are having trouble codes or truly horrendous running behavior (clearly running on 4 or 5 cylinders), an ECU issue is pretty unlikely luckily. Just about any problem you have with those is going to result in dropping entire cylinders or losing complete ability to run.