my rpm fluctuates during idle (especially when engine not warmed up yet)

[dna]

In the last few months, I have noticed that my engine fluctuates, on occasion, during idle. If it does happen, I find it normally happens within the first 15 minutes of being started. My normal idle rpm is 600. When I get this fluctuate, the rpm fluctuates from 600rpm to 400rpm, then back to 600rpm and back to 400, etc, etc, etc. Once I drive off, everything is fine until the next red light. Then I may or may not get this. In a very few rare cases, the engine has stalled too due to the low 400 rpm.

Other than that, engine is strong and no other signs of problems.

Can someone give me some suggestions to what the problem may be? Could my plugs need replacement? Could it be the plug wires?

Thanks