WC beat me to the timing belt possibility. Darn you Josh!!!

Your engine is DOHC, I'd suspect that as it's a fairly common thing to have happen after head work is done. Do a compression test, and that will tell you what the valves are doing and if they're seated right.

I worked on a Mercury Mistake once that had been put back together a tooth off, and 3 of the 8 exhaust valves had melted little v-shaped grooves closest to the pistons. Needless to say it didn't run anymore. If your car feels like it keeps getting slower, I'd definitely pull the timing cover and check it soon.