Engine "pauses" and idling is uneven

[tudorv]

Hi everyone! This is my first post here so bare with me. I tried finding some info on my problem but I could only find bits and pieces in other people's posts.

In short - I bought a 2004 gz250 in November and was only able to ride it for about 1000 miles before winter hit hard. I never took the bike to a mechanic - I figured I would do a full service in the spring.
However, I noticed that the engine doesn't run very smooth when idle. It's barely noticeable, but it goes up and down, up and down. And also from time to time it "pauses". It's hard to explain this. If the engine is cold, it dies when this happens. If it's hot.. it sort of resumes normal idling a second later. It ONLY does this when idling. Other than this, the engine runs fine. Pulls ok. No backfires.

A friend said that most likely the timing chain is loose. But the bike has only 10000 miles on it. Coult this be right? Or maybe dirty carburetor? Valve timig? Spark plug?

Where should I start to get to the bottom of this?

Thanks a lot!

Tudor