Question about my rpms?

I noticed that while at a red light my rpm sometimes drops from the normal 600 down to about 500, maybe close to 400, for a split second then goes up. It happens often enough to be annoying. Any suggestions. 98 sr5 with 82000 miles, never had a tune up, replaced timing belt at 70,000 miles.

Give it new plugs, PCV valve, air filter etc. In short, catch up with any maintenance.