Bicycle Mechanics - New bike, shifting becoming sloppy: diagnosis?

New bike with Ultegra10. Shifted very crisply when new, 600 miles later, shifting has become somewhat sloppy. Downshifts are sometimes accompanied by the chain "sliding" as it tries to catch the larger cog; upshifts are generally noisier and clunkier than before; and the drivetrain is overall noisier. It still hits all 10 gears in back, and things in front seem fine. Cable stretch? Time for a tune-up? What's your call?

New bike with Ultegra10. Shifted very crisply when new, 600 miles later, shifting has become somewhat sloppy. Downshifts are sometimes accompanied by the chain "sliding" as it tries to catch the larger cog; upshifts are generally noisier and clunkier than before; and the drivetrain is overall noisier. It still hits all 10 gears in back, and things in front seem fine. Cable stretch? Time for a tune-up? What's your call?You probably get the first checkup free, but it's most likely cable tension.

Yup, cable tension. After the first few hundred miles, the cables tend to stretch a bit. Take the bike in for a tuneup, or do it yourself - good instructions at www.parktool.com in the repair help section.

Up the tension.

I don't think cables "stretch" in any meaningful way. I believe that it is just the cables seating in their clamps.

Go to the rear deralleur, and turn the nut one-quarter turn COUNTER-clockwise. This will retract the housing from the deralleur, causing it to take up more cable and consequenty upping the tension. You may need to turn it more than one-quarter turn, adjust and take it for a test ride.

Also, have you cleaned the drive train. After cleaning the chain, cassette, chainrings, and deralleurs, the shifting on my bike became much better. My Sora bike shifts incredibly smoothly with a clean drivetrain...

Yep, I've been cleaning the drivetrain regularly (about every 100-150 miles), so I don't think that's an issue.

This was a mailorder bike, so no free LBS checkup :(

Thanks for the advice everyone.