I have a 1999 Mitsubishi Montero with a little over 220,000 miles on it. It runs great and I have no issues with it, except when... If I don't drive it for a week or more and then take it for a trip I get maybe 2-4 miles without issues and then suddenly it starts to misfire. The misfire gets so bad that I have to pull over and stop. If I let it idle it seems to run normally but anything over maybe 1000-1500 rpm causes misfire. If I turn it off and leave it for something like 30 minutes and then start it up again it runs fine and I have no further issues.

It's only happened three times now so details may not be exactly correct, but each time it seems to fix itself and then runs fine until the next time I leave it parked.

At first I thought I had moisture in the ignition system and figured that it dried out after running and then the wait. The second time I thought that maybe the alarm system was somehow causing the issue but now I'm thinking that maybe it's the ECU. Is it possible that it's loosing its memory because an on-board battery has died?

Has anyone experienced something similar or know what's happening?

Any insight would be much appreciated.