Vacuum is the first place to look, since lines and gaskets dry up with time. If you can get a vacuum gauge (they're cheap) hook it up and take a look. At idle, the vacuum should be steady. If it fluctuates, or is low (your average engine will pull 15-20 mmHg depending on condition and/or cam/valvetrain) then you have a vacuum leak. You can also use the gauge to diagnose things like sticky valves (another potential problem on vehicles that sit) or plugged exhaust (vermin make nests in the strangest places). All of these things are easy to check. The starter fluid test is a good one. I prefer to use propane from a small torch, since I can control the flow/spray better. If you haven't already, check for codes. A persistent vacuum leak will usually trigger a code 44 (lean) because too much air is entering the system.
If everything checks out on the vacuum end, you'll want to look at your sensors and make sure everything is functioning properly. While a carb engine will do everything it can to not run, an FI engine will attempt to adjust as much as it can to keep running. This can sometimes mask problems.