Is this the same car that has the head gasket failure and the weird oil situation?
You have five main areas to pursue:
- Intake of air (which includes inspecting/cleaning/replacing air filter)
- Intake of gas (including fuel filter)
- Output of exhaust (clogged converter could mess things up for you)
- Compression (leakdown test can verify any issues)
- Ignition
Plugs, cap/rotor only addresses the last point, ignition. By the way, did you compare your old plugs to a spark plug chart to see if you could diagnose any issues that way?
With the history you have on this car I wouldn't be surprised if it's something major! But might try out smaller stuff first.
- Check for the presence of any trouble codes
- Run a bottle of Red Line SI-1 in the gas tank and do a lot of highway driving to see if it improves. Don't bother with SeaFoam.
- Replace air filter
- Use the 3M Fuel Injector Cleaner Kit - especially the intake cleaner that comes with tubing to insert in throttle body. Again, don't bother with SeaFoam.
- Ensure you have no vacuum leaks by inspecting all the vacuum hoses
- Replace the PCV valve.
- You mentioned cleaning the cap and rotor - how old are those parts? Might have to replace them
- Check resistance in spark plug wires
All of these items are a bit of a shot in the dark without further diagnosis, such as a leakdown test. But they are relatively cheap, and most of them count for periodic maintenance anyhow.