What causes my Toyato to idol rough? It starts easy keeps running smooth no ligh

What is your question?

No lights on runs smooth on road. It just idols rough, what could be causing this? I have been told I need a timing belt and water pump?????

What makes this problem better or worse?

Nothing, hot or cold it idols rough

How long has your 1999 Toyota RAV4 had this problem?

I purchased the vehicle a month ago, it was doing this when I bought it