A weird problem???

[BWatson]

My 300D Starts and runs perfectly until it warms up and then it idles a little rough. If I shut it off and go into a store, It's hard to start when I come out. I did have a recent valve adjustment and I've heard that valves being too tight can cause this, but it didn't start this behavior until 500 or so miles after the adjustment. I plan to replace the fuel filters and see what happens. Other than that, the car has plenty of power warm or cold....I'm clueless. I brought it to my mechanic and it imediately stopped behaving badly so he couldn't tell either.