I have always messed with cars, and often get asked for adive from others.. so Ill offer this advice...
If your car gets over 100K miles its going to start needing more than just regular service & tunup.
So if you dont have the resources and ability to do your own troubleshooting and repairs, then you need to replace it.
As for your misfire issue, there are other tests that also need to be run before you throw money at it.
at 240K I would seriously suggest a compression test & leakdown test.
You could have poor compression in more than one cylinder - no compression = misfire
Those two tests at a reputable mechanic will run you around $3-400, but the tool to do the job can be bought at harbour freight for $19.99.
