Your engine sounds like it's running rich. Have you looked at the air filter to see if it's wet? Something simple and stupid like that could cause the problem. While you're there, pull the entire intake off and look for holes, tears or even mouse nests.

 

Have you lost any coolant since you have owned the car?Could be the water pump could be dripping onto the opti,causing some problems.These symptoms really sound plug and wire related.(high rpm miss).Do the cheap stuff first.Or get it scanned at a dealer.I would replace the coil wire first(weak link),then I would pull #8 plug and see if it is a gunked up or not,and see what plugs the previous owner put in there.Hope this helps.Here is a long shot before you do anything,might spray the mas sensor with a little throttle body cleaner ever so gently,and see if that makes a difference.Hope this helps.

 

Does the car have this issue all the time or only if it's hot?

I would do a little general maintenance before you start suspecting the cat or other real mechanical failure. Pull a couple of plugs and see how they look. Inspect the spark plug wires to see if they're burned, frayed or brittle. And lastly, replace the fuel filter if you don't know when it was last changed.