Do you have a code reader? Is the check engine light on? That's where you need to start or you'll be chasing your tail and throwing money away fixing things that aren't broken. Sounds like something simple to me. Perhaps a spark plug is loose or one of the boots isn't on tight? The shaking idle sounds like a miss and the variable idle could be a loose or torn vacuum hose or both. Coil pack giving up? Is the fuel filter original? A worn cam or cam follower is rare enough that I doubt you've been bitten by it. Broken valve spring is more likely and once again, almost unheard of with this engine. I wouldn't flush the engine. No reason to dilute your oil unless you know for sure it's gummed up. Be logical in your search and eliminate one thing at a time and soon it will be clear where the problem is.

Les